Make the sources be arbitrary generators than just gen's to be more scalacheck idiomatic.

use more scalacheck than rand for generation
Merge branch 'develop' of github.com:twitter/summingbird into feature/use_arbitrary_in_tests

Conflicts:
	summingbird-core/src/test/scala/com/twitter/summingbird/TestGraphGenerators.scala
Use Arbitrary properly
Merge pull request #249 from twitter/feature/use_arbitrary_in_tests

Feature/use arbitrary in tests