CLI: compile preloader against JDK 1.6

To be able to detect the Java runtime version at startup to report an
error if it's < 1.8

compiler/preloader/preloader.iml

@@ -1,12 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <module type="JAVA_MODULE" version="4">
-  <component name="NewModuleRootManager" inherit-compiler-output="true">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_6" inherit-compiler-output="true">
     <exclude-output />
     <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" />
   </component>
-</module>
-
+</module>