With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

I changed the cipherKeysAreRandomlyGenerated test so that it does not compare the new Cipher to the length of the old Cipher.