Removed unused variable
Switched to parsing test files with the json gem, not json schema

In case we ever have bugs in our parsing code
Stopped shadowing the "test" variable name inside the loop
Merge pull request #197 from iainbeeston/remove_unused_var

Tidied the common test suite tests
Made sure that all of the test gemfiles are loading all dev dependencies

As things were (with all dev dependencies defined in the Gemfile) the
additional gemfiles in ./gemfiles (used for testing optional
dependencies) were not requiring additional devdependencies, such as
minitest (they were still passing, but were probably using a different
version of minitest to the main test runs)
Switched on quirks mode for json gem

This makes it behave:

1. More like multi json and yajl, which can parse scalar values (not
just hashes or arrays)
2. More correctly, in terms of the latest json rfc (rfc7159)*

Incidentally both multi_json and rails turn on quirks mode by default.

But there are a few other reasons for doing this - before, if json schema
was passed a string, then parsing would fail and the string would be
passed into the validator as a string. But that string could contain an
integer or a float value. That's clearly not the correct behavior.
Let's use webmock as described in the readme

Instead of requiring it lots of times, and calling it when we don't need
to
Added a "parse_data" option, which can be set to false to skip any
attempt to parse the data