hooks: regression fix of introduced deadlock.

* `Popen.wait` may hang. Lars did it correctly the first time...
  https://docs.python.org/2/library/subprocess.html#subprocess.Popen.wait

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Merge pull request #25 from greut/no-wait

hooks: regression fix of introduced deadlock.