update current-readtable in #%module-begin so that #fn works in repl

see https://github.com/greghendershott/rackjure/issues/45
One reason not to do this is that if a rackjure module is required from a non-rackjure module, it will still update the current-readtable, so the current-readtable in the non-rackjure module would be affected.