Correctly fail hook when rubocop is not installed

When rubocop is not installed, or when the installed version does not
match the version expected by Bundler, the 'pre_commit/rubo_cop' hook
passes 100% of the time because 'result.stdout' is empty.

This commit updates the hook to fail if anything is present in stderr
after the hook runs.

Author: fixlr

lib/overcommit/hook/pre_commit/rubo_cop.rb

@@ -10,6 +10,7 @@ class RuboCop < Base
     def run
       result = execute(command, args: applicable_files)
       return :pass if result.success?
+      return [:fail, result.stderr] unless result.stderr.empty?
 
       extract_messages(
         result.stdout.split("\n"),

spec/overcommit/hook/pre_commit/rubo_cop_spec.rb

@@ -48,5 +48,16 @@
 
       it { should fail_hook }
     end
+
+    context 'when there is an error running rubocop' do
+      before do
+        result.stub(:stdout).and_return('')
+        result.stub(:stderr).and_return([
+          'Could not find rubocop in any of the sources'
+        ].join("\n"))
+      end
+
+      it { should fail_hook }
+    end
   end
 end