Reduce glibmm minimum version to 2.39.93
Reduce glibmm minimum version to 2.39.93

Instead of 2.40.0. This makes gobby work on Ubuntu 14.04 (LTS). See
also #56.

gobby/gobby,dagopoot/gobby,gobby/gobby,dagopoot/gobby,dagopoot/gobby,gobby/gobby