adding python directive in header to allow megasky execution as an executable

git-svn-id: https://svn.ligo.caltech.edu/svn/bayeswave/tags/O2_offline@821 c56465c9-8126-4a4f-9d7d-ac845eff4865

postprocess/skymap/megasky.py

+#!/usr/bin/env python
 import matplotlib
 matplotlib.use('Agg')
 import numpy as np