Fix generated spec_helper so we don't get double formatters.

The spec_helper generated by `rspec --init` could
cause both the progress and documentation formatter
to be used for a command like:

rspec path/to/spec.rb --format progress

This happened because formatters are loaded _after_
`--require` files, so the `config.formatters.none?`
check returned true even though a formatter had
been explicitly set at the command line. It's important
that requires come first so that custom formatters can
be appropriately loaded.