Add base64 gem as dependency; bump minimum Ruby version to 2.7

Author: pilaf

aws-cognito-srp.gemspec

@@ -20,9 +20,10 @@ Gem::Specification.new do |spec|
   end
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = '>= 2.4.0'
+  spec.required_ruby_version = '>= 2.7.0'
 
   spec.add_dependency "aws-sdk-cognitoidentityprovider"
+  spec.add_dependency "base64"
 
   spec.add_development_dependency "bundler", "~> 2.2"
   spec.add_development_dependency "rake", "~> 13.0"