Upgrade to proguard-maven-plugin 2.3.1

Closes gh-233

Author: philwebb

pom.xml

@@ -60,12 +60,12 @@
 				<plugin>
 					<groupId>com.github.wvengen</groupId>
 					<artifactId>proguard-maven-plugin</artifactId>
-					<version>2.0.14</version>
+					<version>2.3.1</version>
 					<dependencies>
 						<dependency>
-							<groupId>net.sf.proguard</groupId>
+							<groupId>com.guardsquare</groupId>
 							<artifactId>proguard-base</artifactId>
-							<version>5.3</version>
+							<version>7.0.0</version>
 							<scope>runtime</scope>
 						</dependency>
 					</dependencies>