Use bundled readline library to prevent system readline from causing issues

The introduction of Python3 and readline caused builds to fail due to an undefined HIST_ENTRY in mysqlclient. Note this only occurred in an EE build because MySQL support is only present in EE.

This config option goes away in MySQL 5.6.5, so this may not be an issue in later versions. We may have to install libreadline6-dev or libedit-dev.

Closes #1482 (closed)