fixed urlsafe base64 encoding

This pull request should fix the url safe base64 encoding. I have noticed that `+` and `/` signs are not being replaced since following refactoring: https://github.com/progrium/ruby-jwt/commit/881e307cfd976642f9468c33bcd0ef3ff5a06c6f

The condition was the wrong way around.

/cc @threedaymonk @ollyjshaw