(maint) Fix travis

* Pin chefstyle to ~> 0.5.0 to avoid bringing in rubocop 0.52.1 with its new checks
* Upgrade system gems to fix bundler in RVM 2.5.0

This fixes issues with the regression tests resolving to the incorrect
bundler, cauing this error with the 2.5.0 RVM:

  1) regression test with cookbook users should match expected output
     Failure/Error: command("git clone -q https://github.com/chef-cookbooks/#{name}.git .")

     Mixlib::ShellOut::ShellCommandFailed:
       Expected process to exit with [0], but received '1'
       ---- Begin output of bundle exec git clone -q https://github.com/chef-cookbooks/users.git . ----
       STDOUT:
       STDERR: /home/travis/.rvm/gems/ruby-2.5.0/bin/bundle:23:in `load': cannot load such file -- /home/travis/.rvm/rubies/ruby-2.5.0/lib/ruby/gems/2.5.0/gems/bundler-1.16.1/exe/bundle (LoadError)
       	from /home/travis/.rvm/gems/ruby-2.5.0/bin/bundle:23:in `<main>'
       	from /home/travis/.rvm/gems/ruby-2.5.0/bin/ruby_executable_hooks:15:in `eval'
       	from /home/travis/.rvm/gems/ruby-2.5.0/bin/ruby_executable_hooks:15:in `<main>'
       ---- End output of bundle exec git clone -q https://github.com/chef-cookbooks/users.git . ----
       Ran bundle exec git clone -q https://github.com/chef-cookbooks/users.git . returned 1
     # ./vendor/bundle/ruby/2.5.0/gems/mixlib-shellout-2.3.2/lib/mixlib/shellout.rb:293:in `invalid!'
     # ./vendor/bundle/ruby/2.5.0/gems/mixlib-shellout-2.3.2/lib/mixlib/shellout.rb:280:in `error!'
     # ./vendor/bundle/ruby/2.5.0/gems/rspec-command-1.0.3/lib/rspec_command.rb:111:in `block in command'
     # ./vendor/bundle/ruby/2.5.0/gems/rspec-command-1.0.3/lib/rspec_command.rb:108:in `tap'
     # ./vendor/bundle/ruby/2.5.0/gems/rspec-command-1.0.3/lib/rspec_command.rb:108:in `command'
     # ./spec/regression/regression_spec.rb:11:in `block (4 levels) in <top (required)>'
     # ./vendor/bundle/ruby/2.5.0/gems/rspec-command-1.0.3/lib/rspec_command.rb:47:in `block (2 levels) in <module:RSpecCommand>'
     # ./vendor/bundle/ruby/2.5.0/gems/rspec-command-1.0.3/lib/rspec_command.rb:45:in `block in <module:RSpecCommand>'

.travis.yml

@@ -13,7 +13,7 @@ branches:
 
 # this avoids build issues with rainbow on old bundler/rubygems
 before_install:
-  #- gem update --system
+  - gem update --system
   - gem --version
   - rvm @global do gem uninstall bundler -a -x -I || true
   - gem install bundler

Gemfile

@@ -8,7 +8,7 @@ group :test do
   gem "minitest", "~> 5.3"
   gem "minitest-reporters"
   gem "simplecov", "~> 0.8"
-  gem "chefstyle", "~> 0.5"
+  gem "chefstyle", "~> 0.5.0"
 end
 
 group :development do