Prepare update to grunt 0.4.2 + update reqs.

Require request (instead of relying on yeoman-generator's request) and
lodash (removed as a dep. in grunt 0.4.2) as modules.
Update grunt to 0.4.2 and bump grunt modules to latest
Compress .gif with imagemin

As req. for imagemin has been bumped, we can process gif with it.
The "cache: false" option is because there's a bug in imagemin 0.5.0:
https://github.com/gruntjs/grunt-contrib-imagemin/issues/140
0.4.2