Additional enforcer rule to verify no dependencies have a bytecode hi…

…gher than 1.8 (#587)

Addresses issue #585

pom.xml

@@ -249,10 +249,35 @@
                                 <requireMavenVersion>
                                     <version>[3.0.4,)</version>
                                 </requireMavenVersion>
+
+                                <!-- Verify that there are no classes in dependencies having bytecode versions
+                                     higher than the minimum JDK version we support.
+                                 -->
+                                <enforceBytecodeVersion>
+                                    <maxJdkVersion>1.8</maxJdkVersion>
+                                </enforceBytecodeVersion>
                             </rules>
                         </configuration>
                     </execution>
                 </executions>
+                <dependencies>
+                    <dependency>
+                        <groupId>org.codehaus.mojo</groupId>
+                        <artifactId>extra-enforcer-rules</artifactId>
+                        <version>1.3</version>
+                    </dependency>
+
+                    <!-- Explicitly set version as a workaround for issue 
+                         https://github.com/mojohaus/extra-enforcer-rules/issues/131.
+                         Can be removed once extra-enforcer-rules is upgraded to a 
+                         version higher than 1.3.
+                     -->
+                    <dependency>
+                        <groupId>org.apache.maven.shared</groupId>
+                        <artifactId>maven-dependency-tree</artifactId>
+                        <version>2.2</version>
+                    </dependency>
+                </dependencies>
             </plugin>
             <plugin>
                 <groupId>org.codehaus.mojo</groupId>