rvm plugin fix #217

Fix for #217 
The rvm plugin copies a load of rvm environment variables to a temporary file (rvm.fish.xxxxxx) by greping for a regex. In newer versions of rvm, this line is included by that regex
```rvm_debug=() {  (( ${rvm_debug_flag:-0} )) || return 0;
 if rvm_pretty_print stderr; then
 printf "%b" "${rvm_debug_clr:-}$*${rvm_reset_clr:-}\n";
 else
 printf "%b" "$*\n";
 fi 1>&2
}```

The fix below removes this line from the grep.