Uncap pbr version

This reverts commit 9414322,
as the workaround for readthedocs seems to be no longer necessary.

Signed-off-by: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

setup.py

@@ -26,5 +26,5 @@
 
 ryu.hooks.save_orig()
 setuptools.setup(name='ryu',
-                 setup_requires=['pbr<1.0'],
+                 setup_requires=['pbr'],
                  pbr=True)