compile with java 8

A PR will be using it, and MC now uses Java 8.

pom.xml

@@ -8,8 +8,8 @@
     <artifactId>GriefPrevention</artifactId>
     <version>16.7.1</version>
     <properties>
-        <maven.compiler.source>1.6</maven.compiler.source>
-        <maven.compiler.target>1.6</maven.compiler.target>
+        <maven.compiler.source>1.8</maven.compiler.source>
+        <maven.compiler.target>1.8</maven.compiler.target>
     </properties>
     <repositories>
         <repository>