Improve compatibility with older Xtext versions

Older Xtext versions did not declare the hard dependency from xtext.java
to jdt.core. To work around this we add the lowest compatible version of
jdt core to the runtime classpath and let Gradle do conflict resolution in
case a newer Xtext version needs a newer JDT version.

gradle.properties

@@ -1,4 +1,4 @@
 minimumXtextVersion = 2.9.0
 #We can't use 2.11 while we still want Java 6 support
 bootstrapXtextVersion = 2.9.1
-latestXtextVersion = 2.11.0.beta2
+latestXtextVersion = 2.11.0.RC1

xtext-gradle-builder/build.gradle

@@ -1,9 +1,12 @@
 description 'A small wrapper for Xtext to make it easier to use from Gradle'
 
 dependencies {
+	compile project(':xtext-gradle-protocol')
 	compile "org.eclipse.xtext:org.eclipse.xtext:$minimumXtextVersion"
-	compile "org.eclipse.xtext:org.eclipse.xtext.java:$minimumXtextVersion"
-	compile "org.eclipse.xtext:org.eclipse.xtext.xbase:$minimumXtextVersion"
 	compile "org.eclipse.xtext:org.eclipse.xtext.smap:$minimumXtextVersion"
-	compile project(':xtext-gradle-protocol')
+	compile "org.eclipse.xtext:org.eclipse.xtext.xbase:$minimumXtextVersion"
+
+	runtime "org.eclipse.xtext:org.eclipse.xtext.java:$minimumXtextVersion"
+	//workaround for old Xtext versions not declaring this hard dependency
+	runtime "org.eclipse.jdt:org.eclipse.jdt.core:3.10.0"
 }