Yara service should use pip to install yara-python

[mrichard91]

When running the bootstrap for yara service on ubuntu it grabs the yara package using apt-get which is currently 1.7.7. We can move this to yara-python in requirements.txt which supports 3.5.

This is the command currently used in ubuntu_install():
sudo apt-get install -y --fix-missing yara

[mgoffin]

Yes! Now that it's properly separated that would be great.