Update Neovim, YCM and Python
Updates:
- Neovim to v0.2.0
- Python2 to 2.7.13
- Python3 to 3.6.0
- YCM to 6ba6b56fb79f5cf2fe07437f09a6cad0f164c0f9
- LLVM to 4.0
Fixes:
- Hides all executable, only expose nvim
- Explicitly set the python executable path for YCM engine