Use JDK 1.6 to compile modules under "core"

Outputs of these modules go into kotlin-stdlib and kotlin-reflect, both
of which must be usable on Java 6

core/util.runtime/util.runtime.iml

@@ -5,9 +5,8 @@
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
     </content>
-    <orderEntry type="inheritedJdk" />
+    <orderEntry type="jdk" jdkName="1.6" jdkType="JavaSDK" />
     <orderEntry type="sourceFolder" forTests="false" />
     <orderEntry type="library" exported="" name="kotlin-runtime" level="project" />
   </component>
-</module>
-
+</module>