switch yajl-ruby to and use unicorn in startup

Was seeing segfaults with the JSON gem, with some content with
interesting chars:

prepare, respond and recover. This British Red Cross led project is
about ‘Wor
Readme update
Switch unicorn in dev to shotgun/thin
Merge pull request #63 from alphagov/unicorn-yajl

Use Thin in development and Yajl for JSON parsing