Fix up some issues in the installation docs.
Fix up some issues in the installation docs.

This change fixes up some nits in the Linux installation documentation
related to various dependencies. This also removes the suggestion to
install using `yum` for CentOS/RedHat, since the EPEL version is lagging
pretty far behind.

Testing Done:
Built docs and checked the output.

Reviewed at https://reviews.reviewboard.org/r/10539/

reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard