Fix "couldn't parse YAML" error after upgrading Bundler

If you just upgraded to Bundler 10.0.10 you might get the following error when bringing up Rails:

Copy/usr/lib/ruby/1.9.1/psych.rb:148:in `parse': couldn't parse YAML at line 17 column 14 (Psych::SyntaxError)

This is caused by Rails localization files (en.yml, de.yml, etc.) using symbols for various translation strings, and Bundler 10.0.10 defaults to a new YAML engine Show archive.org snapshot which cannot handle symbols.

You can switch back to the old YAML engine by putting these lines at the top of your config/boot.rb:

Copyrequire 'yaml'
YAML::ENGINE.yamler= 'syck'