Allow failure of py27-subprocess tox-env while on pytest-2.7

"inprocess by default" was introduced in 2.8, and the current travis
configuration was backported from master so this specific
tox-env will remain disabled while pytest-2.7 maintenance branch
lasts.

.travis.yml

@@ -20,7 +20,9 @@ env:
     - TESTENV=py27-trial
     - TESTENV=py33
     - TESTENV=py33-trial
-    - TESTENV=py27-subprocess
+    # inprocess tests by default were introduced in 2.8 only;
+    # this TESTENV should be enabled when merged back to master
+    #- TESTENV=py27-subprocess
     - TESTENV=doctesting
     - TESTENV=py27-cxfreeze
     - TESTENV=coveralls