Listen for close event rather than exit

Listening to the exit event leads to occasional data loss.

See nateps/connect-gzip#18

lib/runner.js

@@ -59,7 +59,7 @@ exports.compile = function(input, options, callback) {
   compiler.stderr.on('data', function(data) {
     return stderr += data;
   });
-  compiler.on('exit', function(code) {
+  compiler.on('close', function(code) {
     var error;
     if (code !== 0) {
       error = new Error(stderr);