Fix date format

DateTime needs require 'date', but Date is not loaded automatically in
2.0.0-rc1

e.g. railties
$ bundle exec rake test
/Users/sane/.rbenv/versions/2.0.0-rc1/lib/ruby/gems/2.0.0/bundler/gems/ruby-prof-c9d2c0ac109e/ruby-prof.gemspec:56:in
`block in <main>': uninitialized constant DateTime (NameError)