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.

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.

We’re upgrading SimpleCov to 0.13.0 so let’s update the rest of our
gems while we’re at it.

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.

SimpleCov 0.12.0 brings some moderate speed improvements and soon (sadly
not yet) can be switched back to a stable release. Update to matching
Coveralls release, fortunately with fewer dependencies than before.

We must bump coveralls otherwise we get a conflicting dependency: we use
a patched simplecov v0.11.1 but coveralls 0.8.9 needs simplecov 0.10.x.

Closes Homebrew/homebrew#47185.

Signed-off-by: Xu Cheng <xucheng@me.com>

Closes Homebrew/homebrew#41890.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Closes Homebrew/homebrew#31986.
References Homebrew/homebrew#31981.