Add missing require for String#first

When we run `rake deploy:<env>` the Rails environment hasn't been loaded
so ensure that we load the core extension for `String#first`. Previously
it was being implicitly required by another gem.

lib/package_builder.rb

@@ -1,4 +1,5 @@
 require 'tmpdir'
+require 'active_support/core_ext/string/access'
 require 'active_support/core_ext/string/strip'
 require 'aws-sdk-codedeploy'
 require 'aws-sdk-s3'