Add python_requires to help pip

Author: hugovk

setup.py

@@ -39,6 +39,7 @@
         where='.',
         exclude=('test_elasticsearch_dsl*', )
     ),
+    python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*',
     classifiers = [
         "Development Status :: 4 - Beta",
         "License :: OSI Approved :: Apache Software License",