Original solutions
Replaced reduce with all?. Made divisable_by? private. Left an empty line after private.
Replace % with remainder
Rework prime_factors to be more readable
Replace x..y with x.upto(y). Add new line after private.
Replace split('') with chars. Make it so it passes skeptic.
Make frequencies more readable by removing "result" and using Hash[]
Fixes a bug with drop_every that the test cases don't cover. It occurs when the array has repeatable elements. Removes the need for public divisable_by? in Integer
Refactors combine_with to do literally what the task says, making it more readable. Removes the need for the private method.