Update to support SCL Python 3.6 (rh-python36)

- Add 'rh-python36' as default for version "" and "3"
- Allow 'rh-python35' for RHEL 6 (remove special case)
- Update tests accordingly
- Fixes poise#125

test/spec/python_providers/scl_spec.rb

@@ -54,6 +54,16 @@
     it_behaves_like 'scl provider', 'rh-python36'
   end # /context with version "3"
 
+  context 'with version "3.5"' do
+    let(:python_version) { '3.5' }
+    it_behaves_like 'scl provider', 'rh-python35'
+  end # /context with version "3"
+
+  context 'with version "3.4"' do
+    let(:python_version) { '3.4' }
+    it_behaves_like 'scl provider', 'rh-python34'
+  end # /context with version "3"
+
   context 'with version "3.3"' do
     let(:python_version) { '3.3' }
     it_behaves_like 'scl provider', 'python33'