Specify encoding as UTF-8 when reading JSON

This isn't necessary in dev, or when running from the command line on a
preview box, but in the live preview environment, some pound signs in a
schema file were causing a:

    Encoding::InvalidByteSequenceError: "\xC2" on US-ASCII

error when loading the schema json (in schema_config.rb, "load_json")

We're not sure why there's a difference; current theories are that it's
due to something missing in locale data.  Rather than investigate right
now, we're just going to specify that the file should be read as UTF-8.
Merge pull request #275 from alphagov/read-json-as-utf8

Specify encoding as UTF-8 when reading JSON