Support python 3.6 via SCL

[wolf31o2]

Originally filed at poise/poise-languages#12

Attempting to install Python 3.6 using:

python_runtime '3.6' do
  provider :scl
end

Fails with an error message:

* python_runtime[3.6] action install

Running handlers:
[2018-07-11T21:48:36+00:00] ERROR: Running exception handlers
[2018-07-11T21:48:36+00:00] ERROR: Running exception handlers
Running handlers complete
[2018-07-11T21:48:36+00:00] ERROR: Exception handlers complete
[2018-07-11T21:48:36+00:00] ERROR: Exception handlers complete
Chef Client failed. 3 resources updated in 23 seconds
[2018-07-11T21:48:36+00:00] FATAL: Stacktrace dumped to /opt/kitchen/cache/chef-stacktrace.out
[2018-07-11T21:48:36+00:00] FATAL: Stacktrace dumped to /opt/kitchen/cache/chef-stacktrace.out
[2018-07-11T21:48:36+00:00] FATAL: Please provide the contents of the stacktrace.out file if you file a bug report
[2018-07-11T21:48:36+00:00] FATAL: Please provide the contents of the stacktrace.out file if you file a bug report
[2018-07-11T21:48:36+00:00] ERROR: No SCL repoistory package for centos 7.5.1804
[2018-07-11T21:48:36+00:00] ERROR: No SCL repoistory package for centos 7.5.1804
[2018-07-11T21:48:36+00:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)
[2018-07-11T21:48:36+00:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)

Setting this to 3.5 works. I've filed there here, due to the error "ERROR: No SCL repoistory package for centos 7.5.1804" coming from this repository's code.

Can we have 3.6 added to the list at lib/poise_python/python_providers/scl.rb please?

[jflemer-ndp]

I opened a pull request for this (#129)

[jflemer-ndp]

@coderanger I'm not sure why one of the CI tests failed, it doesn't seem related to the change. (And two tests timed out). Do you have any feedback on this PR or the Travis results?