🔧 Relax gem 'oauth2' dependency requirement to be able to install gem 'oauth2 v2'

Author: WaKeMaTTa

omniauth-oauth2.gemspec

@@ -3,7 +3,7 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "omniauth-oauth2/version"
 
 Gem::Specification.new do |gem|
-  gem.add_dependency "oauth2",     "~> 1.4"
+  gem.add_dependency "oauth2",     [">= 1.4", "< 3"]
   gem.add_dependency "omniauth",   [">= 1.9", "< 3"]
 
   gem.add_development_dependency "bundler", "~> 2.0"