Remove and ignore ruby version files
Set the job script name earlier

This allows dispatching to the proxy classes during job configuration.
Add support for custom input formats

This adds an InputFormatProxy along the same lines as the other proxies.

Unfortunately, I couldn’t really figure out how to write a meaningful
unit test, so I implemented a (rather contrived) custom input format in
the integration tests to at least make sure the code runs.

A limitation is that the splits returned from the input format must be
standard Hadoop splits, as any input split must be serialisable and
that would require a significant amount of code.