test: run elixir tests, but down fail entire build if they fail

[janl]

My compromise suggestion for #1902

[eiri]

Failure in tests:

* test GET /dbname/_design_docs (565.1ms)

  1) test GET /dbname/_design_docs (PartitionDDocTest)
     test/partition_ddoc_test.exs:159
     Assertion with == failed
     code:  assert length(body["rows"]) == 1
     left:  0
     right: 1
     stacktrace:
       test/partition_ddoc_test.exs:168: (test)

and green Travis, so works as expected.

That'd be +1 from me to immediately unblock release pipe, but I think we need @wohali opinion on this as well.

[garrensmith]

We can close this since the elixir tests are merged in