Travis::CommitCommand: DRY the rspec [1 of 2]

This patch does not affect behavior.

It uses rspec shared examples so an author can easily
add new commit commands and reuse existing unit tests.
Travis::CommitCommand: DRY up implementation [2 of 2]

Support all forward and backwards forms of command keywords
with a DRYer implementation.

Behavior change: After this commit, recognize the special
"colon" form both forwards and backwards.
IOW both [ci:keyword] and [keyword:ci].