Modify Travis configuration to pre-install blurb.

Author: larryhastings

.travis.yml

@@ -42,6 +42,7 @@ matrix:
         # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
         # (Updating the version is fine as long as no warnings are raised by doing so.)
         - python -m pip install sphinx~=1.6.1
+        - python -m pip install blurb
       script:
         - make check suspicious html SPHINXOPTS="-q -W -j4"
     - os: linux