Lower requests dependency version as 2.4.x break a lot of things

E.g. python-pip is broken with requests 2.4.x. 

On Ubuntu 14.04 the system package python-requests 2.2.1 is installed as dependency of python-pip. With this patch the system package provided requests 2.2.1 satisfies the nagios-mesos dependency and hence pip and nagios-mesos can co-exist on the system.