ignore node_modules folder and force exit code 1 on error to fail build
add JS test runner to travis script list
Add largest series product exercise for elixir
Add ocaml zipper exercise
Add test case to Haskell zipper

This tests the `up` function as well, and ensures that the user has made zippers
instances of `Eq` and `Show`.

This also fixes the type sig in the stub to match what's expected.
Merge pull request #1123 from pminten/ocaml-zipper

OCaml zipper
Merge pull request #1118 from wilmoore/ignore-node-modules

ignore node_modules folder and force exit code 1 on error to fail build
Merge pull request #1119 from wilmoore/add-js-test-to-ci-build

add JS test runner to travis script list
Merge pull request #1122 from petehuang/add-largest-series-product-elixir

Add largest series product exercise for elixir