Make non-https protocols work in host parameter (for GH Enterprise)

[trhodeos]

Creating a Git client for a non-https protocol fails. For instance, using this configuration (with github-site-plugin):

<configuration>
  <host>http://github.internal</host>
  <message>Building site for ${project.version}</message>
</configuration>

fails with the message:

[INFO] Error creating blob: http

This change makes https the default (like before), but supports http (and other protocols, but they should never really be used though..) as well.

[kevinsawicki]

Thanks for contributing this, I've merged it into master.