Incorporate @sapariduo's feedback for Centos 6.4 install.

https://github.com/NYCComptroller/Checkbook/issues/26#issuecomment-34296699
is the source for these documentation changes.

* INSTALL.md
  (Installation): List 'yum install php-xml' for CentOS 6.4.
  (Troubleshooting): New section, with draft @sapariduo notes.