If no syntax errors are found, `gofmt -l % 1>/dev/null` prints the file name to STDOUT, which is redirected to /dev/null. Less to parse for syntastic! If errors are found, they are written to STDERR. This invocation does not alter the source code, which has been indicated in the comments.
Convert the tabs to spaces so that they are counted towards the window width and the status message does not wrap.
Query RVM for the version of ruby that should be used to do the syntax check. This wont work if the user has different versions of ruby for different directories (e.g. if they are using a project rvmrc) as it will only a single ruby version per vim instance. I think this should satisfy almost everyone though, so we can wait for feedback before doing anything more hardcore.