Upgrade bundler

petrify.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 1.15"
+  spec.add_development_dependency "bundler", "~> 2.1"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_runtime_dependency "haml", "~> 5.0"
 end