tox: Update py to 1.4.31.

tox.ini

@@ -25,7 +25,7 @@ deps =
     Mako==1.0.3
     parse==1.6.6
     parse-type==0.3.4
-    py==1.4.30
+    py==1.4.31
     pytest==2.8.3
     pytest-bdd==2.15.0
     pytest-catchlog==1.2.0
@@ -152,7 +152,7 @@ deps =
     -r{toxinidir}/requirements.txt
     apipkg==1.4
     execnet==1.4.1
-    py==1.4.30
+    py==1.4.31
     pyflakes==1.0.0
     pytest==2.8.3
     pytest-cache==1.0
@@ -168,7 +168,7 @@ deps =
     apipkg==1.4
     execnet==1.4.1
     pep8==1.6.2
-    py==1.4.30
+    py==1.4.31
     pytest==2.8.3
     pytest-cache==1.0
     pytest-pep8==1.0.6
@@ -184,7 +184,7 @@ deps =
     apipkg==1.4
     execnet==1.4.1
     mccabe==0.3.1
-    py==1.4.30
+    py==1.4.31
     pytest==2.8.3
     pytest-cache==1.0
     pytest-mccabe==0.1