Cache/fallback for schema URLs in JRubyXML JARVValidators

[ryanfb]

Currently, if the URL goes down, this can cause tests (or a new server instance) to bubble up Java::JavaIo::IOException errors.

We should handle this gracefully instead. Potentially we could cache the schemas in Git and fallback to that, or maybe we could fallback to e.g. the Wayback Machine?

[ryanfb]

This can also result in a Java::JavaNet::ConnectException being raised from the verifier_factory.compileSchema calls in lib/jruby_xml.rb (called from e.g. Identifier#is_valid_xml?).

[ryanfb]

See also #117 (Use schema URL used for validation in data templates)