Also bump requirement in the docs.

Author: abergeron

doc/installation.rst

@@ -21,7 +21,7 @@ Build Requirements
  - (optional) libcheck (check_) to run the C tests.
  - (optional) python (python_) for the python bindings.
  - (optional) mako (mako_) for development or running the python bindings.
- - (optional) Cython >= 0.21 (cython_) for the python bindings.
+ - (optional) Cython >= 0.25 (cython_) for the python bindings.
  - (optional) nosetests (nosetests_) to run the python tests.
 
 Run Requirements