Run flake8 after unit tests on Travis.

Travis will still run tests and upload results to Scrutinizer if there
is a code style error.
Use &encoding for msgpack Packer

- This commit disables automatic encoding of Unicode strings until
  &encoding is retrieved, and then resets the packer to use &encoding
- Previously the msgpack Packer always encoded Unicode as utf-8
Merge pull request #42 from equalsraf/tb-encoding

Use &encoding for msgpack Packer
Merge pull request #43 from fwalch/travis-flake

Run flake8 after unit tests on Travis.
Travis: Use Neovim nightly build.