Update for Pelican v3.7

Fixes #33
diff --git a/requirements.txt b/requirements.txt
index 19bbaa4..7e336cd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
-webassets==0.12.1
 cssmin==0.2.0
-pyScss==1.3.4
+ply==3.10
+pyScss==1.3.5
 slimit==0.8.1
-ply==3.9
+webassets==0.12.1