Test the collection of tests

This is to workaround pytest issues #149 / #180 where distributed
testing doesn't error out if it fails to collect tests.

.travis.yml

@@ -1,6 +1,7 @@
 language: python
 
 env:
+  - TOXENV=collect
   - TOXENV=py26
   - TOXENV=py27
   - TOXENV=py32

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py26,py27,py32,py33,pypy
+    collect,py26,py27,py32,py33,pypy
 
 [testenv]
 deps =
@@ -12,6 +12,10 @@ deps =
 commands =
     py.test -n 8
 
+[testenv:collect]
+commands =
+    py.test --collectonly
+
 [testenv:py32]
 commands =
     py.test