hard require git-{commit,rebase}-mode

There really is no point in delaying telling users that they do need
`git-commit-mode` until they actually try to make a commit, because
that's like the most fundamental git operation from a user perspective.
:-)

Likewise tell users how haven't installed it yet that they need to get
`git-rebase-mode` already at startup.  Interactively rebasing isn't as
important as committing but it's important enough to make it a hard
dependency too.

Update .travis.yml (hope that works on the first try) and README.md
(while also adding some other missing dependency information).