Display warning message in BundleCheck hook

The post-checkout `BundleCheck` hook would return a warning status but
not display a message if the dependencies were out of date.

Change it to actually display a warning message.

Change-Id: I064b5777108967d3e571e46e48d31dd64282d197
Reviewed-on: http://gerrit.causes.com/36485
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: Shane da Silva

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * Add pre-commit `TravisLint` hook which uses the
   [travis-lint](https://github.com/travis-ci/travis-lint) gem
+* Display actual warning message when dependencies aren't satisifed in
+  post-checkout `BundleCheck` hook
 
 ## 0.7.0
 

lib/overcommit/hook/post_checkout/bundle_check.rb

@@ -7,13 +7,17 @@ def run
         return :warn, 'bundler not installed -- run `gem install bundler`'
       end
 
-      return :warn if dependencies_changed? && !dependencies_satisfied?
+      if dependencies_changed? && !dependencies_satisfied?
+        return :warn, "#{LOCK_FILE} is not up-to-date -- run `bundle check`"
+      end
 
       :good
     end
 
   private
 
+    LOCK_FILE = 'Gemfile.lock'
+
     def dependencies_changed?
       result = execute(%w[git diff --exit-code --name-only] + [new_head, previous_head])