Don't leave CMD variable set in environment

As the file is designed to be sourced within a login shell, the $CMD
variable will hang around after its been used if we don't unset it.

bundler-exec.sh

@@ -80,3 +80,5 @@ for CMD in $BUNDLED_COMMANDS; do
         alias $CMD="run-with-bundler $CMD"
     fi
 done
+
+unset CMD