Fix stack overflow in jgem command

In jruby 1.7.10 (possibly earlier) the script in bin/gem simply calls bin/jgem. The jruby formula overwrites jgem with gem in the 'prefix with j' paragraph. This way the jgem script end up calling itself infinitely causing a stack overflow crash.

This patch only overwrites if the target file does not yet exist; otherwise it removes the non-prefixed file.