Backwards incompatible changes

- Drop Python 3.4 support
- Drop Elm 0.18 support
- Require `--fake-license` flag to be specified when invoking elm-doc

New behaviors

- Elm 0.19 is the new & only Elm version supported
- Create an `.elm-doc` directory at the root of the target project
- Do not require any C extension to be built when installing elm-doc
