Remove supervisor from installed packages

Since commit 842e12cf307f1ecd44913025f2c18819a221560e, supervisor is no longer used: mysqld is launched directly, supervisor configurations have been deleted and usages of supervisor have been removed from scripts.

I guess supervisor is not necessary and shouldn't be installed in the apt-get command.

(I noticed it because I wrote an tiny [article](http://pierre-jean.baraud.fr/blog/2014/05/19/deeper-look-dockerfile/) about your image )