Adjust hash syntax for ruby 1.8.7 compatibility

The newer hash syntax (key: value vs. :key => value) is incompatible
with ruby 1.8.7 and thus with anything deployed on Amazon Web Services.
Since the only way of configuring instances with AWS is by using Chef I
would strongly suggest to retain 1.8.7 compatibility, at least until
Amazon moves to using 1.9.x within its Chef environment.

There are no downsides to using the old syntax. It still works with ruby
2.x and 1.9.x.

Reference: https://forums.aws.amazon.com/thread.jspa?threadID=140115