Travis CI: Build documentation in isolated environment

This prevents loading the `sphinx_multiversion` module from old branches
instead of the latest one (due to Python's behaviour of preferring
modules from the CWD).

.travis.yml

@@ -91,6 +91,6 @@ before_script:
 script:
 - mkdir html
 - git fetch --all
-- sphinx-multiversion -W docs html
+- python -I -m sphinx_multiversion -W docs html
 - python setup.py build sdist bdist_wheel