[FIXED JENKINS-24592] Support HTTP proxies

[JENKINS-24592](https://issues.jenkins-ci.org/browse/JENKINS-24592)

This pull request enables the plugin to work behind HTTP proxies.

* TODO
    * [x] [github-api](https://github.com/kohsuke/github-api) 1.59 is released
        * [x] kohsuke/github-api#124
    * [x] [github-api-plugin](https://github.com/jenkinsci/github-api-plugin/) 1.59 is released
    * [x] Remove `-SNAPSHOT` in pom.xml in this pull request