Add a test for Sphinx errors or warnings
Fix some Sphinx warnings
PEP8
Print reason when building docs fails

Print the reason when the building the docs test fails because Sphinx
exits with non-zero. Should help debug this test on Travis..
Don't use subprocess.check_output()

It isn't in Python 2.6
Print reason if getting CKAN version in docs fails

Trying to debug a problem building the docs on Travis..
Don't crash docs build if `git tag` fails

For some reason `git tag -l` seems to be outputting nothing on Travis,
which makes the docs build crash on Travis.

Instead of crashing just use an empty string for the CKAN version, which
will break the docs but no one reads the docs that Travis builds anyway.
Typo
Move check_output() to shared function

Remove two duplicated check_output() functions.

This also fixes a Python 2.6 error that when the subprocess exited with
non-zero check_output() was calling CalledProcessError with an argument
that doesn't exist in 2.6.
Fix a typo in a docstring
Fix a Sphinx error on Travis
Fix a Sphinx error on Travis
Minor tweak to Sphinx test
Merge pull request #1826 from ckan/add-sphinx-test

[#1826] Add sphinx test