Fixes "global name 'p' is not defined" in get_pfile

Traceback (most recent call last):
  File "./sublime_plugin.py", line 200, in on_selection_modified
  File "./sublime_plugin.py", line 154, in run_timed_function
  File "./sublime_plugin.py", line 199, in <lambda>
  File "./tern.py", line 32, in on_selection_modified
  File "./tern.py", line 75, in get_pfile
NameError: global name 'p' is not defined

This fix actually solved my problems with my RequireJS modules :)