Pynvim 0.3.2

Cleanup after the pynvim rename, as well as a few bugfixes and
improvements.

Changes since 0.3.1:

• bfb5c26 support del vim.current.line
• 408025b Handle nvim_error_event (errors from async requests)
• 14ab154 Don't crash when subprocesses write to stdout
• c37b4ed 22e5919 Continue pynvim rename
• 2aa29f9 test with Python 3.7 in Travis.