Add support for remote urls with a `ssh://` prefix.

The Bitbucket documentation sometimes tell the user to specify the remote url for the repo as `ssh://git@bitbucket.org:user/repo.git` (for example, https://confluence.atlassian.com/display/BITBUCKET/Import+code+from+an+existing+project and https://confluence.atlassian.com/display/BITBUCKET/Use+the+SSH+protocol+with+Bitbucket). This pattern is not handled by the regex at https://github.com/tommcdo/vim-fubitive/blob/master/plugin/fubitive.vim#L15.

Here is a patch with an updated regex to handle these urls.