Require bundler >= 1.5.0 (#30353).

git-svn-id: http://svn.redmine.org/redmine/trunk@17781 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -1,8 +1,6 @@
 source 'https://rubygems.org'
 
-if Gem::Version.new(Bundler::VERSION) < Gem::Version.new('1.5.0')
-  abort "Redmine requires Bundler 1.5.0 or higher (you're using #{Bundler::VERSION}).\nPlease update with 'gem update bundler'."
-end
+gem "bundler", ">= 1.5.0"
 
 gem "rails", "5.2.2"
 gem "rouge", "~> 3.3.0"