travis: Support multiple example directories
travis: Support multiple example directories

This serves a few purposes:

* Validating that a test suite can accept multiple possible type
  signatures, such as anagram in #393.
* Isolating the `package.yaml` for each example, preventing us from
  having to show students our example's `package.yaml`.
* (With a future extension that marks an example as should-fail) allows
  us to make sure our test suite catches certain solutions and causes
  them to fail.

Closes #395

exercism/xhaskell