Re-install Travis Gem at 1.6.11
Enable Git 2.0's automatic commit signing

See previous comments on this topic for context, and see the relevant
sections from the git 2.0.0 release notes [1].

This commit also re-introduces the `git logsig` alias for convenient
display of signed commits.

[1]: http://article.gmane.org/gmane.comp.version-control.git/250341