Use SSH private keys for RemoteGithubBuild#postCloneSetup()

[anttiviljami]

Contribution Type: bug fix Link to Intent to Implement: https://github.com/Block8/PHPCI/issues/1133#issuecomment-215366679 Link to Bug: https://github.com/Block8/PHPCI/issues/1133

This pull request affects the following areas:

• [ ] Front-End
• [x] Builder
• [ ] Build Plugins

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the contributing guidelines?
• [x] I have checked that another pull request for this purpose does not exist.
• [x] I have considered, and confirmed that this submission will be valuable to others.
• [x] I have created or updated the relevant documentation for this change on the PHPCI Wiki.
• [ ] Do the PHPCI tests pass?

Detailed description of change:

Use SSH private keys for RemoteGithubBuild#postCloneSetup()

Applies @bdanchilla's patch, which fixes #1133