Add the libsass version to --version

[asottile]

Resolves #140

$ sassc --version
sassc 0.10.1 (sass/libsass 3.3.3)

[coveralls]

Coverage increased (+0.04%) to 78.277% when pulling 65d63db on asottile:add_libsass_version into 0f04ecd on dahlia:master.

[coveralls]

Coverage increased (+0.04%) to 78.277% when pulling c4e4d9a on asottile:add_libsass_version into 0f04ecd on dahlia:master.

[dahlia]

Merged!