make travisCI happy with bundler version bump

Author: jspaleta

CHANGELOG.md

@@ -15,6 +15,7 @@ Which is based on [Keep A Changelog](http://keepachangelog.com/)
 - Bonsai Asset enablement. Making changes to travis config to enable Bonsai asset building during release deployment
 
 ### Changes
+- Update development dependency: bundler ~> 2.1
 - Update development dependency: codeclimate-test-reporter ~> 1.0
 - Update development_dependency: github-markup ~> 3.0
 - Update development_dependency: rake ~> 12.3

sensu-plugins-zookeeper.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_runtime_dependency 'sensu-plugin', '~> 4.0'
   s.add_runtime_dependency 'zookeeper',    '1.4.11'
 
-  s.add_development_dependency 'bundler',                   '~> 1.7'
+  s.add_development_dependency 'bundler',                   '~> 2.1'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 1.0'
   s.add_development_dependency 'github-markup',             '~> 3.0'
   s.add_development_dependency 'pry',                       '~> 0.10'