Add EqualsHashCode check to checkstyle config

- Enables checkstyle verification that hashCode() is overridden if equals(Object) is overridden (and vice versa)

.checkstyle.xml

@@ -159,6 +159,7 @@
         <module name="EmptyCatchBlock">
             <property name="exceptionVariableName" value="ignore|ignored"/>
         </module>
+        <module name="EqualsHashCode"/>
     </module>
     <module name="FileTabCharacter"/>
 </module>