remove duplicated rspec devel dependency

When installing billy/master using Bundler, it raises the following warning:
```
puffing-billy at /home/stefan/.rvm/gems/ruby-2.1.0/bundler/gems/puffing-billy-382f9a407315 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on rspec (>= 0, development), (>= 0) use:
    add_runtime_dependency 'rspec', '>= 0', '>= 0'
Using puffing-billy (0.2.2) from git://github.com/oesmith/puffing-billy.git (at master)
```

I presume the attached patch should fix the issue.