Travis: never send emails on success

This silences passing notifications for new branches
Switch to minitest for consistent output between Ruby versions

In Ruby 2.0, `test/unit` activates a fancy output style for terminals
that doesn't look well in Travis logs. I could disable it with the
`--jobs-status=normal` test optiont, but that option isn't supported in
Ruby 1.8.7.

Instead of dealing with inconsistencies between test support in
different Ruby versions, I'm simply switching to a fixed version of
Minitest.
Update Cucumber for better Minitest compatibility

Otherwise it hits deprecation warnings because it accesses the
Minitest::Unit module.
Merge branch 'minitest'
Have Travis install tmux and zsh prior to the build
Run shell completion tests in an isolated tmux instance

Makes these tests work when executed outside of tmux as well.

Adds tmux 1.6 compatibility. The tests should now work with tmux 1.6-1.9
Detect tmux and run shell completion tests
Account for various locations of bash/zsh completion scripts

On different systems, bash/zsh completion scripts for git are installed
in various places. Account for that by searching the list of known paths.

In zsh mode, if a git-distributed zsh completion script can't be found
(e.g. on Ubuntu), ignore that and let it fall back to zsh-distributed
completion script.
Compensate for variable sorting in bash shell completion

Fixes https://travis-ci.org/github/hub/jobs/13920941
Increase timeout while waiting for shell prompt

Attempt to fix https://travis-ci.org/github/hub/jobs/13973437
Increase Aruba process exit timeout

Attempt to fix https://travis-ci.org/github/hub/jobs/13973842
Merge pull request #429 from github/travis-tmux

Run shell completion tests on Travis CI