Bump json-schema version to 2.2.3

This is to pick up our fix to the gem for symbol keys in hashes in
arrays:

    https://github.com/hoxworth/json-schema/pull/105

This means that we can write our test data more idiomatically as ruby
hashes with symbol keys. While working on the schema we had found a
workaround to this which involved calling .to_json on the data before
validating it, but the fix makes this unnecessary.
Merge pull request #18 from alphagov/bump_json-schema_version

Bump json-schema version to 2.2.3
Add examples of JSON

These are the manual and some sections of the scraped Employment Income Manual
from manuals-frontend, modified to fit our initial schema.

- EIM00100.json is the 'About' section, with content in the body and no child sections
- EIM77010.json is an appendix, with more content in the body and no child sections
Remove 'manual' from key names

It seems redundant when you have the context of the format name.
Simplify (shorten) example JSON
Strawman of JSON to send to Content Store
Remove some sections for easier maintenance

We don't really need so many examples, and it was becoming a hassle to tweak
and maintain whilst keeping them consistent.
Move JSON closer to Content Store format

This will simplify the job of translation.
Add a schema for manuals

Updated tests to continue to use valid test data.
Add a schema for manual sections

Updated tests to continue to use valid test data.
First tests of Manual schema
Basic specs for Section schema
Remove workaround for symbol keys in test data

This now works as expected in the gem.