Gemfile: only list 'rspec' once

This is to resolve this warning from bundler:

    Your Gemfile lists the gem rspec (>= 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.
Merge pull request #86 from docwhat/bundle-warning

Gemfile: only list 'rspec' once