fix path to bower

Gruntfile.js

@@ -95,7 +95,7 @@ module.exports = function(grunt) {
         var done = this.async();
         var child_process = require('child_process');
         var exec = child_process.exec;
-        exec('bower update && cd ./components/strophe && make normal',
+        exec('./node_modules/.bin/bower update && cd ./components/strophe && make normal',
              function (err, stdout, stderr) {
                 if (err) {
                     grunt.log.write('build failed with error code '+err.code);