Remove trailing slash when using snakebite rename

Old functionality caused odd behavior when doing:

    snakebite.rename('foo/', 'bar/')

and the bar did not exist. As result it created the 'bar' first and
then moved 'foo' in there, resulting in 'bar/foo'.

By stripping the slashes on the right, the behavior works as one would
think: renames 'foo' to 'bar'.
Merge pull request #507 from nailor/strip-trailing-slash-snakebite-rename

Remove trailing slash when using snakebite rename