Elixir Docs: Add line to compile the source exs file

We must compile the elixir source before we're able to execute the tests.

```text
~/code/exercism/elixir/bob$ elixir bob_test.exs
** (FunctionClauseError) no function clause matching in ExUnit.DocTest.extract_from_moduledoc/1
    (ex_unit) lib/ex_unit/doc_test.ex:351: ExUnit.DocTest.extract_from_moduledoc(nil)
    (ex_unit) lib/ex_unit/doc_test.ex:342: ExUnit.DocTest.extract/1
    (ex_unit) lib/ex_unit/doc_test.ex:167: ExUnit.DocTest.__doctests__/2
    bob_test.exs:6: (module)

~/code/exercism/elixir/bob$ elixirc bob.exs
~/code/exercism/elixir/bob$ elixir bob_test.exs
# it works!
```