Revert "remove guava dependency because it seems not to be needed"

This reverts commit 359f591

.classpath

@@ -2,6 +2,7 @@
 <classpath>
 	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8"/>
 	<classpathentry kind="src" path="src"/>
+	<classpathentry kind="lib" path="lib/guava-18.0-sources.jar"/>
 	<classpathentry kind="lib" path="lib/java-json.jar"/>
 	<classpathentry kind="lib" path="lib/org.eclipse.elk.alg.common.compaction-0.3.0-sources.jar"/>
 	<classpathentry kind="lib" path="lib/org.eclipse.elk.alg.common.compaction-0.3.0.jar"/>