update test requirements

requirements should be kept up to date to ensure we get bug fixes and new features as they come - particularly py.test, which we were running an 18 month old version for, and missing out on some useful xfail and fixture enhancements, among other things

requirements_for_test.txt

@@ -1,11 +1,10 @@
 -r requirements.txt
-pep8==1.5.7
-pytest==2.8.3
-pytest-mock==0.8.1
-pytest-cov==2.2.0
+pep8==1.7.0
+pytest==3.0.1
+pytest-mock==1.2
+pytest-cov==2.3.1
 coveralls==1.1
-mock==1.0.1
-moto==0.4.19
-flex==5.7.0
-freezegun==0.3.6
-requests-mock==0.7.0
+moto==0.4.25
+flex==5.8.0
+freezegun==0.3.7
+requests-mock==1.0.0