fix build issue #50

[ratmice]

I merely tested that the build succeeded, but haven't gone over the output yet.

About the change to sphinx.directives.CodeBlock, we can see in the following commit that it appears to have been deprecated in favor of sphinx.directives.code.CodeBlock and then subsequently removed in a new version.

https://github.com/sphinx-doc/sphinx/commit/00002397764b391b0ebf737bbe47202ed4c98fff

[FranklinChen]

Thanks, seems to work for me.