Make checkstyle fail build by default

It seems almost every developer has caused Travis CI to break due to
checkstyle failures. To prevent further breakages, checkstyle will now
fail the build by default.

Specifying checkstyle.ignoreFailures=true in ~/.gradle/gradle.properties
will turn the failures into warnings, which was the old behavior.

build.gradle

@@ -94,7 +94,7 @@ subprojects {
     checkstyle {
         configFile = file("$rootDir/checkstyle.xml")
         toolVersion = "6.5"
-        ignoreFailures = true
+        ignoreFailures = false
         if (rootProject.hasProperty("checkstyle.ignoreFailures")) {
             ignoreFailures = rootProject.properties["checkstyle.ignoreFailures"].toBoolean()
         }