Fixes: Global config and github-api versioning

### Description
* Setting the plugin's global configuration was failing due to not being able to extract the form data for `unstableAs`. It is now fixed.
* There was a version conflict between `com.coravy.hudson.plugins.github.github 1.4` and the plugin that was directly depending on `org.jenkins-ci.plugins.github-api 1.35`. `github 1.4` has a transitive dependency on `github-api 1.28`. Jenkins picks up the **v1.28 instead of v1.35** which breaks some of the API calls. Changed according to suggestion found [here](https://wiki.jenkins-ci.org/display/JENKINS/GitHub+API+Plugin) so as not to embed the github-api jar and also fix the versioning issue.