Remove EL6 from the scripted installer, stop CircleCI builds

[eorochena]

Removed EL6 from the scripted installer and stop EL6 builds in CIrcleCI, following StackStorm/community#39

This PR doesn't cover rpm adjustments and st2python removal related to EL6 packaging.

[arm4b]

Thanks a lot for your contribution and help removing EL6!

As I understand in this PR we're covering the following sub-tasks from the StackStorm/community#39

• update scripted installer to remove EL6 support
• stop the EL6 build in CircleCI

I updated the original PR description and also provided more feedback in the comments below.
Please also adjust the list of OS supported platforms here:

st2-packages/scripts/st2_bootstrap.sh

Lines 155 to 158 in 5d92c25

	if [[ "$RHMAJVER" != '6' && "$RHMAJVER" != '7' && "$RHMAJVER" != '8' ]]; then
	echo "Unsupported distro version $RHMAJVER! Aborting!"
	exit 2
	fi

[eorochena]

@armab I have implemented or the necessary changes to the PR also all CI checks have passed,.

Thanks for the suggestions.

[arm4b]

Looks good, thanks for the contribution!