Make the compiled package runnable on Java 7 or later

Should fix #34

pom.xml

@@ -40,10 +40,29 @@
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <maven.compiler.source>1.7</maven.compiler.source>
+    <maven.compiler.target>1.7</maven.compiler.target>
   </properties>
 
+  <profiles>
+    <profile>
+      <id>jdk9</id>
+      <activation>
+        <jdk>[1.9,)</jdk>
+      </activation>
+      <properties>
+        <maven.compiler.release>7</maven.compiler.release>
+      </properties>
+    </profile>
+  </profiles>
+
   <build>
     <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.10.1</version>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-source-plugin</artifactId>
@@ -111,15 +130,6 @@
           </execution>
         </executions>
       </plugin>
-      <plugin>
-        <groupId>org.apache.maven.plugins</groupId>
-        <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.1</version>
-        <configuration>
-          <source>6</source>
-          <target>6</target>
-        </configuration>
-      </plugin>
     </plugins>
   </build>