Fix shakapacker_version_as_array caching

Author: alexeyr

lib/react_on_rails/packer_utils.rb

@@ -57,6 +57,7 @@ def self.shakapacker_version
     end
 
     def self.shakapacker_version_as_array
+      return @shakapacker_version_as_array if defined?(@shakapacker_version_as_array)
       match = shakapacker_version.match(ReactOnRails::VersionChecker::VERSION_PARTS_REGEX)
 
       # match[4] is the pre-release version, not normally a number but something like "beta.1" or `nil`