Added Gradle 1.8 to prebuilts.

Needed to compile Android Gradle plug-in.

Change-Id: I264114b9bcf8489b5c4a329e0834b8c0f35ca436
diff --git a/PREBUILT b/PREBUILT
index 2836722..764409a 100644
--- a/PREBUILT
+++ b/PREBUILT
@@ -4,4 +4,5 @@
 http://services.gradle.org/distributions/gradle-1.4-all.zip
 http://services.gradle.org/distributions/gradle-1.5-all.zip
 http://services.gradle.org/distributions/gradle-1.6-all.zip
-http://services.gradle.org/distributions/gradle-1.7-bin.zip
\ No newline at end of file
+http://services.gradle.org/distributions/gradle-1.7-bin.zip
+http://services.gradle.org/distributions/gradle-1.8-bin.zip
\ No newline at end of file