In gradle version 2.14 a gradle class used by this plugin was deprecated and moved to a different internal package.
I committed a fix and published to gradle plugin portal a new version of Task Tree Plugin: 1.2.2.
Tested the fix successfuly on gradle versions 2.14 and 3.0-milestone-2.
Version 1.2.2 of the plugin will only work with gradle versions 2.14+.
Version 1.2.1 of the plugin must be used for gradle vresions 2.3-2.13.