This requires updating to Rubocop 0.49.0 which will require some fixes
to rules, in Homebrew/brew and Homebrew/homebrew-core but opening this
for now so I remember.

Attempt to improve Travis CI. This time without any vendoring gems
nonsense.

This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

This reverts commit 8d948e4b, reversing
changes made to 23728729.

Tweak our Travis CI configuration and the one we suggest for taps to
be faster and use caching.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Travis needs some coaxing to find where we put it. This should improve
build reliability due to network issues.

This will avoid `brew doctor` trying to retap `homebrew/core` which is
both slow and error-prone.

For code coverage from `brew test-bot`.

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>