Update README.md

Removes suggestion that bundler should automatically `require` flowaker. 

[This line](https://github.com/foraker/flowaker/blob/master/lib/flowaker/rspec.rb#L22) blows up unless `rspec-rails` has been loaded as `rspec-rails` provides `Rspec.configuration`.  Usually, `rspec-rails` is loaded first, and there is no issue, but sometimes `flowaker` is loaded first. 

Alternatively, we can add `rspec` and `rspec-rails` to the gemspec.