Speed up license checks

Previously the function _project_is_apache would get called twice for
every line being checked. And this function would read the file
'LICENSE' every time.

Since we don't expect 'LICENSE' to change during the run of hacking,
cache the result.

Using cProfile, _project_is_apache took:
Before: 1.030
After: 0.008

Change-Id: I762ad0f910f59c7844d283b7965b28628d6273b6