v1.2.0: gometalinter support, no more cache

This release includes two PRs: #12 and #15.

#15 fixes retool to support github.com/alecthomas/gometalinter. The retool do command now appends the _tools directory to GOPATH when it runs subcommands, and it sets GOBIN to _tools/bin. In addition, when syncing, retool will preserve the linters vendored by gometalinter in its _linters directory, so retool do gometalinter will work without hassle.

#12 removes uses of retool's cache. This might make things a tad slower when adding lots of tools for the first time, but should eliminate a large class of bugs.