Loosen Bundler dependency to allow both 1.x and 2.x (#591)

Due to Bundler 2.0 shenanigans, Travis was failing; loosening the development dependency on Bundler fixes it.

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **(breaking change)** Upgraded minimum Ruby version to 2.3.7, and upgraded Rubocop to 0.60.0. This additionally changes the name of some public constants. ([#487](https://github.com/meew0/discordrb/pull/487), thanks @ChallahuAkbar)
 - Dependencies for `rbnacl`, `rake`, and `rspec` have been updated ([#538](https://github.com/meew0/discordrb/pull/538), thanks @ChallahuAkbar)
 - The monolithic `data.rb` file has been split into multiple files for easier development ([#482](https://github.com/meew0/discordrb/pull/482))
+- Loosened `bundler` development dependency to allow use of `bundler` 1.x and 2.x ([#591](https://github.com/meew0/discordrb/pull/591), thanks ChallahuAkbar)
 
 ## [3.3.0] - 2018-10-27
 [3.3.0]: https://github.com/meew0/discordrb/releases/tag/v3.3.0

discordrb.gemspec

@@ -33,7 +33,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.3.7'
 
-  spec.add_development_dependency 'bundler', '~> 1.10'
+  spec.add_development_dependency 'bundler', '>= 1.10', '< 3'
   spec.add_development_dependency 'rake', '~> 12.0'
   spec.add_development_dependency 'redcarpet', '~> 3.4.0' # YARD markdown formatting
   spec.add_development_dependency 'rspec', '~> 3.8.0'