Add docker to travis

Author: mxmeinhold

.travis.yml

@@ -1,6 +1,8 @@
 language: python
 python:
   - "3.6"
+services:
+  - "docker"
 
 install:
   - "pip install -r requirements.txt"
@@ -11,3 +13,4 @@ install:
   - "npm install"
 script:
   - "gulp production"
+  - "docker build -t conditional ."