Use cvxopt option to silence newer versions of GLPK

This follows a similar pull request made for the `polytope` package at tulip-control/polytope#11.  It is a simple change made in the commit 6384f106b88025e57496d09bb69cb6f8afe650a0 .  I have tested it in the following configuration:
* Ubuntu 14.04 on Linux x86_64
* Python 2.7.6
* `polytope` 0.1.0
* `cvxopt` 1.1.7
* GLPK 4.52.1, provided by the Debian package libglpk-dev 4.52.1-2build1