Update plutus.gemspec

Should fix the bundler validation message: 
```
plutus at /Users/graeme/.rvm/gems/ruby-2.0.0-p353@cloudsponge/bundler/gems/plutus-9317ff0bb2c9 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-rails (~> 2.6, development), (~> 2.6) use:
    add_runtime_dependency 'rspec-rails', '~> 2.6', '~> 2.6'
```