Add a travis build for the documentation

.travis.yml

@@ -0,0 +1,7 @@
+language: python
+python:
+  - "3.6"
+install:
+  - pip install -r docs/requirements.txt
+script:
+  - sphinx-versioning build -b -B 1.5 -r 1.5 -w '^[0-9.]*$' -w master -W '^$' docs/ build/