add benchmark-ips gem. Require by encryption testcase

dependency introduced
[here](rails/rails@638a92f)

Author: mgrunberg

Gemfile

@@ -10,6 +10,7 @@ gem "bcrypt"
 gem "pg",      ">= 0.18.0"
 gem "sqlite3", "~> 1.4"
 gem "tzinfo-data", platforms: [:mingw, :mswin, :x64_mingw, :jruby]
+gem "benchmark-ips"
 
 if ENV["RAILS_SOURCE"]
   gemspec path: ENV["RAILS_SOURCE"]