Force should be explicitly stated in call.

Automatically setting the force flag to true will cause all sprite maps generated by a compiling file to also be regenerated, even if they haven't been changed, in turn causing compilation to be overly slow. This was especially the case in third party programs that watch project directories on their own and call compass compile on individual files (instead of assuming `compass watch`).

Instead use `compass compile my_file.scss --force`.
Merge pull request #1187 from michaelgilley/master

Force should be explicitly stated in call.