Update the Travis CI config to test doc building

This change updates the .travis.yml configuration to include
building the Sphinx documentation whenever the Travis CI build
is triggered.

.travis.yml

@@ -57,6 +57,14 @@ matrix:
           os: linux
           dist: trusty
           env: TOXENV=bandit
+        - python: 2.7
+          os: linux
+          dist: precise
+          env: TOXENV=docs
+        - python: 2.7
+          os: linux
+          dist: trusty
+          env: TOXENV=docs
 install:
   - pip install tox
   - pip install bandit