Allow checkstyle failures fatal, and fix last issues

Failing disabled by default, but setting checkstyle.ignoreFailures=false
in ~/.gradle/gradle.properties enables failing. Travis will always run
with such failing enabled.
diff --git a/build.gradle b/build.gradle
index c21e1b8..75fed00 100644
--- a/build.gradle
+++ b/build.gradle
@@ -91,6 +91,10 @@
     checkstyle {
         configFile = file("$rootDir/checkstyle.xml")
         toolVersion = "6.2"
+        ignoreFailures = true
+        if (rootProject.hasProperty("checkstyle.ignoreFailures")) {
+            ignoreFailures = rootProject.properties["checkstyle.ignoreFailures"].toBoolean()
+        }
     }
 
     checkstyleMain {