Pin mkdocs-bootswatch and mkdocs-bootstrap to fix build errors.

[richadams]

The release of MkDocs 1.0 has changed some of the dependencies, so our builds now error out because it's trying to install a later version of MkDocs than we can use with our current theme. Installing mkdocs-bootswatch v0.1.0 and mkdocs-boostrap v0.1.1 before trying to install mkdocs satisfies the dependencies so it doesn't attempt to install the newer version that breaks.