download url hot fix

by release of v2.1.0 download url changes as following 

v2.1.0 => http://downloads.typesafe.com/play/2.1.0/play-2.1.0.zip
older versions e.g. 2.0.4 => http://downloads.typesafe.com/releases/play-2.0.4.zip

so i variablized download url depending on version