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/.travis.yml b/.travis.yml
index 5e1889f..058f8b0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,6 +11,8 @@
 
 before_install:
   - buildscripts/make_dependencies.sh # build protoc into /tmp/proto3-a2
+  - mkdir -p $HOME/.gradle
+  - echo "checkstyle.ignoreFailures=false" >> $HOME/.gradle/gradle.properties
 
 jdk:
   - oraclejdk8