Fixed deprecation warnings from Gradle's ExtraPropertiesExtension style change

See: http://gradle.org/docs/current/dsl/org.gradle.api.plugins.ExtraPropertiesExtension.html

Summary of link above:
This (ExtraProperties) mechanism replaces the ability to create new properties on domain
objects directly. In versions of Gradle prior to 1.0-milestone-9, new properties could be
added to objects by assignment:

project.myProp = "myValue"

This functionality has been deprecated (a deprecation warning message will be issued when
this is attempted). Future versions of Gradle will remove this functionality completely,
resulting in an exception being thrown.