Ignore certain files when exporting with git-archive.

The content of the .gitref file will be changed to the SHA-1 of the
commit being archived.

Also, ignore dist/ directory at project root.
Improve Travis CI coverage

* .travis.yml: Cleaner, more LISP combinations, but test only emacs 23 and
emacs 24 on debian.
Closes #90: Merge pull request from branch 'pr/90'

Conflicts:
	ChangeLog