Disable broken GitHub Maven plugin

The upload of the Maven site to GitHub pages fails with the following
messages:

[INFO] --- site-maven-plugin:0.10:site (default) @ tinylog-gelf ---
[INFO] Creating 59 blobs
[INFO] Creating tree with 60 blob entries
[INFO] Merging with tree a8673842eacda89d9d148ad0f6c8ca00a70299c5
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:59 min
[INFO] Finished at: 2014-11-18T15:16:51+01:00
[INFO] Final Memory: 38M/438M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.github.github:site-maven-plugin:0.10:site (default) on project tinylog-gelf: Error creating commit: Invalid request.
[ERROR]
[ERROR] Expected data to be of type "string"; value was: nil.
[ERROR] Expected data to be of type "string"; value was: nil. (422)
[ERROR] -> [Help 1]