Improve .ackrc

Just making some tweaks to my .ackrc to improve my searching experience.
The colors, in particular, will work better with my color scheme than
the defaults.

I removed the .scss type because it appears to be part of ack's default
types now.
Use "light" font smoothing in .osx

This level of font smoothing provides a more useful contrast between
bold and not bold text.
Add "composable" and "changelog" to vim dictionary