Removed Pharo-stable build from TravisCI. It's failing on osx

Author: olekscode

.travis.yml

@@ -15,15 +15,15 @@ smalltalk:
 #  - Squeak-4.5
 
 #  - Pharo-alpha
-  - Pharo-stable
+#  - Pharo-stable
 #  - Pharo-7.0
   - Pharo-6.0
 #  - Pharo-5.0
 #  - Pharo-4.0
 #  - Pharo-3.0
 
 #  - Pharo64-alpha
-  - Pharo64-stable
+#  - Pharo64-stable
 #  - Pharo64-7.0
   - Pharo64-6.0