printrun: add python-numpy dependency

[ryanmjacobs]

plater would not start without Python 2.7 numpy being installed.

ryan@void ~ $ plater
/usr/lib/python2.7/site-packages/wx-3.0-gtk2/wx/_core.py:16629: UserWarning: wxPython/wxWidgets
  warnings.warn("wxPython/wxWidgets release number mismatch")
Traceback (most recent call last):
  File "/bin/plater", line 23, in <module>
    from printrun.stlplater import StlPlater
  File "/usr/lib/python2.7/site-packages/printrun/stlplater.py", line 36, in <module>
    from printrun import stltool
  File "/usr/lib/python2.7/site-packages/printrun/stltool.py", line 23, in <module>
    import numpy
ImportError: No module named numpy