Adjust httpretty inclusion

Although we're removing httpretty, it seems like Gerrit is no longer
honoring our fixgates.txt file, so I've included httpretty back in
test-requirements now that the versions we need to skip are skipped
there.

Change-Id: I246fcbc40570331cce1bff8dc5717fe3bacb5e40

fixgates.txt

@@ -1 +0,0 @@
-httpretty>=0.8.0,!=0.8.1,!=0.8.2,!=0.8.3,!=0.8.7,!=0.8.8

test-requirements.txt

@@ -5,6 +5,7 @@ hacking>=0.9.2,<0.10
 
 coverage>=3.6
 discover
+httpretty>=0.8.4,!=0.8.7,!=0.8.8
 fixtures>=0.3.14
 mock>=1.0
 python-subunit>=0.0.18

tox.ini

@@ -10,7 +10,6 @@ setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-       -r{toxinidir}/fixgates.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]