Upgrade RuboCop 0.37.2 -> 0.39.0 for Travis

This also fixes a bug introduced by change in behavior in `parser`
2.3.0.7. See: whitequark/parser#283

Author: sds

Gemfile

@@ -4,6 +4,6 @@ source 'https://rubygems.org'
 gem 'coveralls'
 
 # Pin RuboCop for Travis builds.
-gem 'rubocop', '0.37.2'
+gem 'rubocop', '0.39.0'
 
 gemspec

lib/overcommit/hook_loader/plugin_hook_loader.rb

@@ -79,7 +79,7 @@ def create_ad_hoc_hook(hook_name)
       # Implement a simple class that executes the command and returns pass/fail
       # based on the exit status
       hook_class = Class.new(hook_base) do
-        def run # rubocop:disable Lint/NestedMethodDefinition
+        def run
           result = @context.execute_hook(command)
 
           if result.success?