Updated Xtext version for Luna target from 2.7 to 2.8

releng/org.yakindu.base.target/Luna.target

@@ -7,7 +7,6 @@
 </location>
 <location includeAllPlatforms="false" includeConfigurePhase="true" includeMode="planner" includeSource="true" type="InstallableUnit">
 <unit id="org.eclipse.gmf.feature.group" version="1.8.1.201408201453"/>
-<unit id="org.eclipse.xtext.sdk.feature.group" version="2.7.3.v201411190455"/>
 <unit id="org.eclipse.jdt.feature.group" version="3.10.1.v20150204-1700"/>
 <unit id="org.eclipse.emf.sdk.feature.group" version="2.10.2.v20150123-0452"/>
 <unit id="org.eclipse.emf.compare.source.feature.group" version="3.0.2.201501201328"/>
@@ -16,6 +15,10 @@
 <repository location="http://download.eclipse.org/releases/luna"/>
 </location>
 <location includeAllPlatforms="false" includeConfigurePhase="false" includeMode="planner" includeSource="true" type="InstallableUnit">
+<unit id="org.eclipse.xtext.sdk.feature.group" version="2.8.4.v201508050135"/>
+<repository location="http://download.eclipse.org/releases/mars"/>
+</location>
+<location includeAllPlatforms="false" includeConfigurePhase="false" includeMode="planner" includeSource="true" type="InstallableUnit">
 <unit id="com.google.inject" version="3.0.0.v201312141243"/>
 <unit id="com.google.inject.source" version="3.0.0.v201312141243"/>
 <unit id="com.google.inject.multibindings" version="3.0.0.v201402270930"/>