Sync requirements from global requirements

spec repo has requirement which are not in sync with g-r.

Incompatible dependency issue can occur due to old versions in
requirements.txt

One example is "Detected a distutils installed project" issue is hitting
for argparse which can be fixed by Ib7e74912b36c1b5ccb514e31fac35efeff57378d
-http://logs.openstack.org/34/269934/1/check/gate-qa-specs-docs/fc7a540/console.html

But to avoid such issue in future its better to sync all requirements with g-r.

This will be needed if check requirements job is going to be added for qa-specs.
Related-Bug: #1536535

Change-Id: Ic85e33c56c94b86ded656ca8405fbb30caeee824

requirements.txt

@@ -1,4 +1,4 @@
-pbr>=0.6,!=0.7,<1.0
-oslosphinx
-sphinx>=1.1.2,!=1.2.0,<1.3
+pbr>=1.6  # Apache-2.0
+oslosphinx>=2.5.0,!=3.4.0  # Apache-2.0
+sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3  # BSD
 yasfb>=0.5.1