Relax version constraints on Sinatra libraries (#680)

To allow using newer versions of Sinatra libraries.
Additionally add gem `rack-test` as development dependency.

Author: ashmaroli

jekyll-admin.gemspec

@@ -31,11 +31,12 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 2.3.0"
 
   spec.add_dependency "jekyll", ">= 3.7", "< 5.0"
-  spec.add_dependency "sinatra", "~> 1.4"
-  spec.add_dependency "sinatra-contrib", "~> 1.4"
+  spec.add_dependency "sinatra", ">= 1.4"
+  spec.add_dependency "sinatra-contrib", ">= 1.4"
 
   spec.add_development_dependency "bundler", ">= 1.7"
   spec.add_development_dependency "gem-release", "~> 0.7"
+  spec.add_development_dependency "rack-test", "~> 1.0"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.4"
   spec.add_development_dependency "rubocop-jekyll", "~> 0.10.0"