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