Allow setting Verdict::Segmenter::FixedPercentageSegmenter.salt

This way you can change how the segmenter assigns subjects to groups. 

This is very useful for test suites, to make sure that tests are not depending on the the segmentation logic being deterministic.
``` ruby
# In test_helper.rb
Verdict::Segmenter::FixedPercentageSegmenter.salt = SecureRandom.uuid
```

The salt will be included in the string, over with the MD5 hash is calculated, which in turn is used to determine in what group the subject is.

@jeromecornet @peterjm @pseudomuto