Update bundler requirement from ~> 1.0 to ~> 2.0

Updates the requirements on [bundler](https://github.com/bundler/bundler) to permit the latest version.
- [Release notes](https://github.com/bundler/bundler/releases)
- [Changelog](https://github.com/bundler/bundler/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bundler/bundler/commits/v2.0.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

split-api.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'sinatra'
   gem.add_dependency 'multi_json'
 
-  gem.add_development_dependency 'bundler',     '~> 1.0'
+  gem.add_development_dependency 'bundler',     '~> 2.0'
   gem.add_development_dependency 'rspec',       '~> 3.8.0'
   gem.add_development_dependency 'rack-test',   '~> 1.1'
   gem.add_development_dependency 'rake',        '~> 12.0'