Add python_requires to setup.py (#2465)

Author: djhoese

setup.py

@@ -69,5 +69,6 @@
       install_requires=INSTALL_REQUIRES,
       tests_require=TESTS_REQUIRE,
       url=URL,
+      python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*',
       packages=find_packages(),
       package_data={'xarray': ['tests/data/*']})