Remove .git suffix on destination directory if URL ends with it.

Hey there,

I ran into an issue when managing some pull requests in my [Homesick Chef cookbook](https://github.com/fnichol/chef-homesick). It looks like for Homesick gem versions greater than 0.7.0 Git URLs ending in `.git` will retain the suffix when the castle directory is created. I believe the regression was introduced by https://github.com/technicalpickles/homesick/commit/376fd88fc956885fb41587c7e70c097e54b174b8.

This pull request cleans up the change in behavior and adds a sanity spec to cover future refactorings.

For example, the following:

    homesick clone git://github.com/technicalpickles/pickled-vim.git

should produce a castle directory of:

    $HOME/.homesick/repos/pickled-vim

Thanks for homesick, essential sanity for me when spinning up new systems :smile: