Merge pull request #2 from jakubjosef/master

Make server.source variable truly optional.

home_assistant/server.sls

   {%- if server.source is defined and server.source.get("engine", "git") %}
   - editable: git+{{ server.source.address }}@{{ server.source.version }}#egg=homeassistant
   {%- else %}
-  - name: homeassistant{%- if server.source.version is defined %}=={{ server.source.version }}{%- endif %}
+  - name: homeassistant{%- if server.get('source', {}).version is defined %}=={{ server.source.version }}{%- endif %}
   {%- endif %}
   - pre_releases: True
   - bin_env: {{ server.dir.base }}
 
 {%- endif %}
 
-{%- endif %}
+{%- endif %}