Ensure that bug number is actually a number for skip_because

This commit adds a regex check that bugs numbers passed into the
skip_because decorator are actually numbers. Previously any string
could be passed into the decorator which defeats the whole purpose of
the decorator.

Change-Id: I437fd8fab8721711530249894fae2c3c13683b4d
Merge "Ensure that bug number is actually a number for skip_because"