Upgrade to Proguard 9.0.7

Closes gh-357

Author: philwebb

pom.xml

@@ -62,12 +62,12 @@
 				<plugin>
 					<groupId>com.github.wvengen</groupId>
 					<artifactId>proguard-maven-plugin</artifactId>
-					<version>2.3.1</version>
+					<version>2.6.0</version>
 					<dependencies>
 						<dependency>
 							<groupId>com.guardsquare</groupId>
-							<artifactId>proguard-base</artifactId>
-							<version>7.0.0</version>
+							<artifactId>proguard-core</artifactId>
+							<version>9.0.7</version>
 							<scope>runtime</scope>
 						</dependency>
 					</dependencies>