(maint) Add rubocop checks to Travis

This commit adds rubocop checks to Travis for PR CI.  Any rubocop failures will
not fail the build yet.  Fixes for rubocop violations will happen in later
commits.

.travis.yml

@@ -3,11 +3,30 @@ sudo: false
 language: ruby
 services:
   - redis-server
-rvm:
-  - 2.1.1
-  - 2.2.1
-  - 2.2.2
-  - jruby-1.7.8
+
+matrix:
+  include:
+    - rvm: 2.3.3
+      env: "CHECK=rubocop"
+
+    - rvm: 2.1.1
+      env: "CHECK=test"
+
+    - rvm: 2.2.1
+      env: "CHECK=test"
+
+    - rvm: 2.3.3
+      env: "CHECK=test"
+
+    - rvm: jruby-1.7.8
+      env: "CHECK=test"
+
+  # Remove the allow_failures section once
+  # Rubocop is required for Travis to pass a build
+  allow_failures:
+    - rvm: 2.3.3
+      env: "CHECK=rubocop"
+  
 # This below is a temporary shim to bypass these bundler+jruby bugs:
 # 
 #   https://github.com/bundler/bundler/issues/4975
@@ -16,4 +35,4 @@ install:
   - gem install bundler --version 1.12.5
   - bundle _1.12.5_ install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle}
 script: 
-  - bundle _1.12.5_ exec rake
+  - "bundle _1.12.5_ exec rake $CHECK"