Fixes issue #366.

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.

- enable compilation during CI build only for those that currently pass.
  (let's tackle the failing tests+example code one at a time).