revert "stick with pbr 0.5.19" commit

revert the following commit because it doesn't work with
the latest oslo.config which requires newer pbr.

|commit 205485e0520d8f37c49b66bbbf012a9448cd9367
|Author: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
|Date:   Fri Aug 30 14:08:49 2013 +0900
|
|    stick with pbr 0.5.19
|
|    the latest versions of pbr generate scripts which don't support
|    multiversion.  to avoid importing modules from older multiversion-aware
|    installations of ryu, we prefer multiversion-aware scripts.
|
|    Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
|    Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

the problem reported by Satoshi Kobayashi on ryu-devel.
avoid pbr's non multi-version aware script

otherwise the latest ryu-manager can pick up older modules
if multiple versions of ryu is installed on a system.