Implement API to fetch compiled version

In reference to https://github.com/sass/libsass/pull/693  
- Add option to show compiled versions
- Update build to match latest libsass master

https://travis-ci.org/mgreter/sassc/builds/43346128
```bash
$ ./bin/sassc -v
sassc: 3.0.2-2-g6cc2
libsass: 3.0.2-66-g63eb
sass2scss: 1.0.3
```