Remove dependencies to fix CentOS install

Also includes explicit python >= 2.6 dep. since we noticed that building
on a system with 2.7 includes this one by default, and CentOS doesn't
have packages for 2.7 so it breaks the build again. If it is ever
re-enabled it'll be safe already.

Fixes MN-297
Signed-off-by: Galo Navarro <galo@midokura.com>