ENYO-3237: restore sendRelease call

The this.sendRelease() call used to send the 'release' event at the
end of a 'hold' was mistakenly removed in commit 331542 which
removed the enyo.pool code. This restores it to its place of glory.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)