www/py-libsass: Update to 0.13.7

Changelog:
https://github.com/sass/libsass-python/releases/tag/0.13.7

Also discovered the problem that it recompiles all
.cpp files again during the 'install' phase:
https://github.com/sass/libsass-python/issues/234

This is a preexisting priblem.

Reported by: portscout
Approved by: tcberner (mentor, implicit)