Use gradle root project build dir in check-license (#144)

Fixed check-license to use root project's build dir property instead of relative path which resolves to user.dir.

Signed-off-by: Usman Saleem <usman@usmans.info>

gradle/check-licenses.gradle

@@ -160,7 +160,7 @@ task checkLicenses {
 
   def bads = ""
   doLast {
-    def xml = new XmlParser().parse('build/reports/license/license-dependency.xml')
+    def xml = new XmlParser().parse("${rootProject.buildDir}/reports/license/license-dependency.xml")
     xml.each { license ->
       if (!acceptedLicenses.contains((license.@name).toLowerCase())) {
         def depStrings = []