fix 'tox -e docs' by limiting sphinx versions

pbr has issues invoking sphinx versions >1.3 (http://pad.lv/1379998)

The fix here is two-fold:
 a.) no longer use 'pip install --pre'
     I'd rather spend less time debugging failures due to new pip packages.
 b.) shorten the list of requirements on sphinx as a result of 'a'
     Since 1.3b* versions were pre, and we're no longer installing with --pre
     we do not need to explicitly blacklist them.

At a future date when pbr is available with a fix, we can remove the upper
limit on sphinx.

Change-Id: I696fc3e4ef8803bbb2a95e34501d5330a64ab93c

test-requirements.txt

@@ -8,7 +8,7 @@ nose
 testtools
 
 # For doc building
-sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3
+sphinx>=1.2.0,<1.3
 oslosphinx
 
 # For style checking

tox.ini

@@ -15,6 +15,10 @@ deps = -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/requirements.txt
 commands = {envpython} {toxinidir}/tools/noproxy nosetests {posargs}
 
+# tox uses '--pre' by default to pip install.  We don't want that, and
+# 'pip_pre=False' isn't available until tox version 1.9.
+install_command = pip install {opts} {packages}
+
 [testenv:py26]
 deps = {[testenv]deps}
     importlib