Fix manifest access.

Implicit use of the jar task does not work
in Gradle 2.0 anymore.

Change-Id: I1efc1f2fafd5d43b1fda00faf705ddf6cbc3a78f
(cherry picked from commit b9c9541421a9b7ffae2b0de6679438741ae9bbfe)
diff --git a/build.gradle b/build.gradle
index d95fb2e..1a061f2 100644
--- a/build.gradle
+++ b/build.gradle
@@ -22,7 +22,7 @@
 		resources.srcDirs = ['res']
 	}
 }
-manifest.from file('manifest.txt')
+jar.manifest.from file('manifest.txt')
 
 dependencies {
 	compile files('lib/apache-ant-1.9.4.jar')