[buildout] Remove branch from beat.backend.python and beat.core

They both have been merged to master.

buildout.cfg

@@ -8,9 +8,9 @@ eggs += ipdb
         autopep8
 
 [sources]
-beat.core = git git@gitlab.idiap.ch:beat/beat.core branch=1.6.x
+beat.core = git git@gitlab.idiap.ch:beat/beat.core
 beat.cmdline = git git@gitlab.idiap.ch:beat/beat.cmdline branch=1.4.x
-beat.backend.python = git git@gitlab.idiap.ch:beat/beat.backend.python branch=1.5.x
+beat.backend.python = git git@gitlab.idiap.ch:beat/beat.backend.python
 beat.examples = git git@gitlab.idiap.ch:beat/beat.examples egg=false
 
 [docker_images]