allow "Current" to fail without affecting badge
export PATH

idiomatic, but not strictly necessary
echo hack to force readable Travis labels
update .travis.yml commentary
recast travis env var as `$CASK_RUBY_TEST_VERSION`

It may not always be a system Ruby if 1.9 and 2.1 are added.
Merge pull request #4760 from rolandwalker/travis_refactor

Travis refactor