Remove ruby-sass requirement (fixes #123)

Change-Id: I933c34d384f22a65e1b737e1e7dc13a20703e171
diff --git a/Dockerfile b/Dockerfile
index 2eff6b4..44003e2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -3,8 +3,7 @@
 
 # This follows the "multi-stage build" pattern
 
-RUN npm install -g sass && \
-    npm install grunt && \
+RUN npm install grunt && \
     npm install -g grunt-cli
 
 # Copy repository respecting .dockerignore