Install cython alongside tox

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index fcb8f8e..603ee94 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,6 +21,6 @@
     - python: pypy
       env: TOXENV=pypy
 
-install: pip install tox
+install: pip install cython tox
 
 script: tox