Check extension module was compiled.

.travis.yml

@@ -7,4 +7,4 @@ install:
     - pip install tox cython
     - cython --cplus msgpack/_packer.pyx msgpack/_unpacker.pyx
 
-script: "tox && MSGPACK_PUREPYTHON=x tox"
+script: tox

tox.ini

@@ -1,8 +1,14 @@
 [tox]
-envlist = py26,py27,py32,py33,py34,pypy,pypy3
+envlist = {py26,py27,py32,py33,py34}-{c,pure},{pypy,pypy3}-pure
+
+[variants:pure]
+setenv=
+    MSGPACK_PUREPYTHON=x
 
 [testenv]
 deps=
     pytest
 
-commands=py.test test
+commands=
+    c: python -c 'from msgpack import _packer, _unpacker'
+    pure: py.test test