Update rack-test requirement from ~> 0.6 to ~> 1.1

Updates the requirements on [rack-test](https://github.com/rack-test/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack-test/rack-test/releases)
- [Changelog](https://github.com/rack-test/rack-test/blob/master/History.md)
- [Commits](https://github.com/rack-test/rack-test/commits/v1.1.0)

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

Author: dependabot-support

split-api.gemspec

@@ -21,6 +21,6 @@ Gem::Specification.new do |gem|
 
   gem.add_development_dependency 'bundler',     '~> 1.0'
   gem.add_development_dependency 'rspec',       '~> 2.6.0'
-  gem.add_development_dependency 'rack-test',   '~> 0.6'
+  gem.add_development_dependency 'rack-test',   '~> 1.1'
   gem.add_development_dependency 'rake',        '~> 12.0'
 end