Removed the delay when hitting `esc` in insert mode.

When hitting `esc` in insert mode, vim pauses for a short amount of
time (~500ms) to wait and see if you are going to press a key
combination using escape. This means there is a noticeable pause
(especially when using powerline) before vim exits insert mode. As I
don't plan to use any escape-based shortcuts, I have simply disabled
this - allowing for immediate exiting from insert mode.