libsass 0.15.0

• Fix invalid escape sequences [#249 by Anthony Sottile]
• Add code of conduct [#251 by Nick Schonning]
• Add support for python3.7 and remove testing for python3.4 [#254 by Anthony Sottile]
• Add strip_extension option for wsgi / distutils builder [#55 #258 by Anthony Sottile #260 by Morten Brekkevold]
• Deprecate sassc (replaced by pysassc). [#262 by Anthony Sottile]
• Import abc classes from collections.abc to remove DeprecationWarning [#264 by Gary van der Merwe #265 by Anthony Sottile]