Fix ruamel_yaml for Travis CI on osx

Author: ischoegl

interfaces/cython/cantera/test/test_reactor.py

@@ -5,7 +5,10 @@
 
 import numpy as np
 from .utilities import unittest
-from ruamel.yaml import YAML
+try:
+    from ruamel.yaml import YAML
+except:
+    from ruamel_yaml import YAML
 
 import cantera as ct
 from . import utilities