Log Message:
Update hoe to 3.15.1.

=== 3.15.1 / 2016-06-15

* 1 bug fix:

  * rake newb does a better job of activating gems it had to install. (duckinator)

=== 3.15.0 / 2016-03-25

* 1 minor enhancement:

  * Added ruby version declarations for hoe-spec. ruby20!

* 2 bug fixes:

  * Fixed running rdoc if it was satisfied outside of GEM_HOME.
  * Relaxed requirement on rake to include new rake 11.x. (hsbt)