Update .jshintrc to not explicitly set es5: true

Doing so generates an error that prevents the default grunt task from being run. The error seems to be caused by this change recently added to jshint 2.0: https://github.com/jshint/jshint/issues/1137

When `"es5": true` is in the `.jshintrc` file, running `grunt` results in this failure:

```
Running "jshint:gruntfile" (jshint) task
Linting Gruntfile.js ...ERROR
>> ES5 option is now set per default

Warning: Task "jshint:gruntfile" failed. Use --force to continue.

Aborted due to warnings.
```