deps: Add lua nvim-client as a dependency
test: Remove run-functional-tests.py

Now that the lua client is available, python/lupa are no longer necessary to run
the functional tests. The helper functions previously defined in
run-functional-tests.py were adapted to test/functional/helpers.lua.
test: Use lua to perform sanity API checks

Sanity API checks made by the python-client in the api-python travis target were
converted to lua and will now live in this repository. This will simplify
performing breaking changes to the API as it won't be necessary to send parallel
PRs the python-client.
Merge PR #1296 'Use the lua client to run functional tests'