Add unit test for removing a dotfile with a missing target

If the remove operation is issued on a dotfile pointing to a target
that's already been removed from the repository, then the symlink should
just be removed quietly.

Re: #25
Verify symlink removal in unit test

Re: #25
Only replace existing dotfiles on remove operation

It is possible that the dotifle is removed from the repository before
the remove is issued.  In this case only the symlink is removed.

Fixes #25
Remove python 2.5 from travis-ci configuration