Only include facon modules once..

When calling `extend Facon::SpecHelpers` multiple times in one test (or one app) I was getting the following error:
```
Terminating app due to uncaught exception 'NameError', reason: 'spec_helpers.rb:7:in `extended:': undefined method `mock:' for class `Should' (NameError)
```
`Facon::Baconize::ShouldExtensions` was causing the issues when trying to undefine methods that were already undefined.

To reproduce this error, uncomment line #4 in lib/motion-facon/spec_helpers.rb and run `rake spec`

Cheers! :beers: