readthedocs.org build fix

[sampullman]

This is a potential fix for #389 which appears to be caused by this Sphinx issue. I can't verify it without running the actual build on Read the Docs, but the problem should go away if the build uses a new Sphinx version.

Note that wiping the environment on readthedocs.org may be required.

The second commit fixes a bunch of Pygments lexer errors that showed up after the upgrade.

[pirapira]

Looks good to me.

A remaining task is to see how to detect this in Travis.

[sampullman]

@paulvi I was not aware that those files were completely off limits. Updating Sphinx, which is required for the Read the Docs build, introduces two build errors in the archive. If the old docs cannot be modified, the solution is probably to remove them from the build process (it's probably better to do this regardless).