OS X Travis CI is particularly error-prone, slow and unnecessary for
us when we're already running (fast) OS X builds on Jenkins so use it
just for Linux testing.

This should verify that we're not breaking our tests on Linux.

We have Jenkins to test the other versions.

Putting the Ruby 1.8 we use to run tests on 10.9 into the PATH adversely
affects formulae with a Ruby dependency (possibly other formulae, too).
Remove this hack, as it is no longer needed.

Closes Homebrew/homebrew#47694.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Closes Homebrew/homebrew#43635.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes Homebrew/homebrew#47056.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Closes Homebrew/homebrew#45816.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

If a PR is edited(e.g. force pushed) during the build.
TRAVIS_COMMIT will be lost. Refetching the merge ref wouldn't
help this situation as well. Since Travis already have reset
$TRAVIS_BUILD_DIR to the commit we need to test, let's use it to
determine TRAVIS_COMMIT directly.

Ref: https://github.com/travis-ci/travis-ci/issues/2666

Closes Homebrew/homebrew#43976.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>