Insights: Use py venv in bower install for sassc

We added a bower post-install task that pre-compiles the edX pattern library in
insights. This requires the executable sassc which is installed in
python-libsass. It was already being installed in a previous ansible task
(install application requirements), but we weren't sourcing the python
virtualenv where it was installed for the bower install task.

playbooks/roles/insights/tasks/main.yml

@@ -63,6 +63,7 @@
 - name: install bower dependencies
   shell: >
     chdir={{ insights_code_dir }}
+    . {{ insights_venv_dir }}/bin/activate &&
     . {{ insights_nodeenv_bin }}/activate && {{ insights_node_bin }}/bower install --production --config.interactive=false
   become_user: "{{ insights_user }}"
   tags: