Add compatibility for Coffeescript 2 (and hubot 3)

[phucnh]

Summary

This PR continues #528 works.
Make sure the classes in this code work in Coffeescript 2 (and with hubot 3).

A few of the semantic differences between Coffeescript 1 classes and ES2015 classes has introduced some backwards compatibility issues, as follows:
Breaking Changes From CoffeeScript 1.x to 2

Known Issue: This PR doesn't make hubot-slack work with hubot 2 in Coffeescript 2. Because hubot 2 still based on Coffeescript 1.

Refs: #526

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[phucnh]

Tests are failed on nodejs <= 5 with envs are HUBOT_VERSION="^3.0" COFFEESCRIPT_VERSION="^2.0".
Seems that because the destructuring assignment syntax: {fork} = child_process;

Test failed detail

> mocha --compilers coffee:coffeescript/register --require coffee-coverage/register-istanbul --reporter spec test
"--compilers" will be removed in a future version of Mocha; see https://git.io/vdcSr for more info
/home/travis/build/slackapi/hubot-slack/node_modules/coffeescript/lib/coffeescript/register.js:61
    ({fork} = child_process);
     ^^^^^^
ReferenceError: Invalid left-hand side in assignment
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/home/travis/build/slackapi/hubot-slack/node_modules/coffeescript/register.js:1:63)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:20:19)
    at /home/travis/build/slackapi/hubot-slack/node_modules/mocha/bin/_mocha:453:3
    at Array.forEach (native)
    at Object.<anonymous> (/home/travis/build/slackapi/hubot-slack/node_modules/mocha/bin/_mocha:445:19)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:148:18)
    at node.js:405:3

[codecov-io]

Codecov Report

Merging #565 into master will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   85.19%   85.42%   +0.22%     
==========================================
  Files           6        6              
  Lines         385      391       +6     
  Branches       84       84              
==========================================
+ Hits          328      334       +6     
  Misses         34       34              
  Partials       23       23              

Impacted Files	Coverage Δ
src/bot.coffee	74.75% <100.00%> (+0.24%)	⬆️
src/message.coffee	90.90% <100.00%> (+0.48%)	⬆️

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[mistydemeo]

@aoberoi Any chance we could get your review here? This would be really useful for me.

[seratch]

@phucnh
Thank you for taking the time to make this PR. This looks great and is really amazing!

@aoberoi @stevengill @shaydewael
I'll verify the details (just in case) again and merge this next week. Also, I would like to work on the next minor version (v4.8) later. I think enabling developers to use CoffeeScript 2 is quite helpful as the major version supports async/await in the language (it's not available in CoffeeScript 1.x!). Let me know if you have thoughts on this.

[seratch]

I've reverted this PR as it's not fully compatible with CoffeeScript 2 yet. I'm still positive to support CoffeeScript v2 in future versions but for the next minor version (v4.8.0 - https://github.com/slackapi/hubot-slack/milestone/4), this improvement may be out of scope.