Disable Gradle linter check for unused dependencies in Travis CI

It's telling me to remove "eclipse-deps:org.eclipse.core.runtime:+"
and "org.osgi:org.osgi.core:4.2.0" as unused dependencies in the
"com.ibm.wala.cast.java.ecj" subproject.  However, these two
dependencies (jar files) are actually needed; compilation fails
without them.

build.gradle

@@ -128,6 +128,7 @@ allprojects {
 		excludedRules = [
 			'duplicate-dependency-class',
 			'transitive-duplicate-dependency-class',
+			'unused-dependency',
 			]
 	}
 }