Do not list rspec as development dependency

Otherwise, Travis will return an error with:

> Your Gemfile lists the gem rspec (~> 2.4.0) more than once.
> You should probably keep only one of them.
> While it's not a problem now, it could cause errors if you change the version of just one of them later.
> You are trying to install in deployment mode after changing
> your Gemfile. Run `bundle install` elsewhere and add the
> updated Gemfile.lock to version control.