Re #1000631. Modified so the compilers are not reset when the classpa…

…th is invalid.

The reset in this case is useless, generates log and was making the tests fail on trully
headless systems.

org.scala-ide.sdt.core/src/scala/tools/eclipse/ScalaProject.scala

@@ -401,10 +401,14 @@ class ScalaProject(val underlying: IProject) extends HasLogger {
   def classpathHasChanged() {
     classpathCheckLock.synchronized {
       try {
-        resetCompilers()
         // mark as in progress
         classpathHasBeenChecked= false
         checkClasspath()
+        if (classpathValid) {
+          // no point to reset the compilers on an invalid classpath,
+          // it would not work anyway
+          resetCompilers()
+        }
       }
     }
   }