bumped cython versions to match

scikit-learn's travis.yml for 0.20.2

Author: florian

.travis.yml

@@ -14,10 +14,10 @@ env:
     - MODULE=category_encoders
   matrix:
     # The versions should match the minimal requirements in requirements.txt and setup.py
-    - DISTRIB="conda" PYTHON_VERSION="2.7" CYTHON_VERSION="0.21"
+    - DISTRIB="conda" PYTHON_VERSION="2.7" CYTHON_VERSION="0.23.5"
       NUMPY_VERSION="1.11.1" PANDAS_VERSION="0.21.1" PATSY_VERSION="0.4.1"
       SCIKIT_VERSION="0.20.2" SCIPY_VERSION="0.17.0" STATSMODELS_VERSION="0.6.1"
-    - DISTRIB="conda" PYTHON_VERSION="3.5" COVERAGE="true" CYTHON_VERSION="0.23.4"
+    - DISTRIB="conda" PYTHON_VERSION="3.5" COVERAGE="true" CYTHON_VERSION="0.25.2"
       NUMPY_VERSION="1.11.1" PANDAS_VERSION="0.21.1" PATSY_VERSION="0.4.1"
       SCIKIT_VERSION="0.20.2" SCIPY_VERSION="0.17.0" STATSMODELS_VERSION="0.6.1"