Update tox config to install libsass and use Django 1.9.1

Author: Andrey Fedoseev

tox.ini

@@ -14,15 +14,16 @@ deps =
     pytest-django
     pretend
     watchdog
+    libsass
     coverage==3.7.1
     pytest-cov==2.1.0
     django16: Django>=1.6,<1.7
     django17: Django>=1.7,<1.8
     django18: Django>=1.8,<1.9
-    django19: Django==1.9rc1
+    django19: Django==1.9.1
 commands = py.test static_precompiler
 setenv =
     PYTHONPATH = {toxinidir}
 
-[testenv:py27-django18]
+[testenv:py27-django19]
 commands = py.test static_precompiler --cov static_precompiler --cov-report xml