Cython version tweak for 3.7

Author: lmcinnes

.travis.yml

@@ -14,7 +14,7 @@ env:
     - MODULE=hdbscan
   matrix:
     - DISTRIB="conda" PYTHON_VERSION="3.7"
-      NUMPY_VERSION="1.15.3" SCIPY_VERSION="1.2.1" CYTHON_VERSION="0.28.6"
+      NUMPY_VERSION="1.15.3" SCIPY_VERSION="1.2.1" CYTHON_VERSION="0.28.5"
     - DISTRIB="conda" PYTHON_VERSION="3.5" COVERAGE="true"
       NUMPY_VERSION="1.10.4" SCIPY_VERSION="0.17.0" CYTHON_VERSION="0.23.5"