Bump elixir's version to 0.14.2

Made it work with elixir v0.14.2

* Fix files which specify elixir version; mix.exs and .travis.yml
* Change the notation of `defexception`
* Correct conversion function (e.g. `integer_to_list` => `Integer.to_char_list`)
* Replace `teardown/2` in tests with `on_exit/2`