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/descriptor.loader.java/descriptor.loader.java.iml

@@ -5,7 +5,7 @@
     <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" name="javax.inject" level="project" />
     <orderEntry type="module" module-name="util.runtime" />