Support Java 8 by upgrading to ProGuard 5.0

Proguard 5.0 which supports Java 8 was released.
http://proguard.sourceforge.net/downloads.html
This patch changes ProGuard version from 4.11 to 5.0

Additionally, I think it is better that proguard-maven-plugin and ProGuard version are matched from the view point of perspicuity.