Fix bundler version

Gemfile.lock

 PATH
   remote: .
   specs:
-    fizzy-api (0.1.4)
+    fizzy-api (0.1.5)
       active_interaction (~> 3.6)
       httparty (~> 0.15)
       virtus (~> 1.0)
@@ -51,7 +51,7 @@ GEM
     factory_bot_rails (4.8.2)
       factory_bot (~> 4.8.2)
       railties (>= 3.0.0)
-    httparty (0.15.6)
+    httparty (0.16.2)
       multi_xml (>= 0.5.2)
     i18n (0.9.1)
       concurrent-ruby (~> 1.0)
@@ -121,7 +121,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  bundler (~> 1.16)
+  bundler (~> 1.15)
   factory_bot_rails
   fizzy-api!
   rake (~> 12.3)

fizzy_api.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'active_interaction',             '~> 3.6'
   gem.add_dependency 'virtus',                         '~> 1.0'
 
-  gem.add_development_dependency 'bundler',            '~> 1.16'
+  gem.add_development_dependency 'bundler',            '~> 1.15'
   gem.add_development_dependency 'rake',               '~> 12.3'
   gem.add_development_dependency 'rspec',              '~> 3.7'
   gem.add_development_dependency 'factory_bot_rails', '~> 4.8'