Convert all Ruby hashes to >= 1.9 sexy syntax

All the hashes with key-value pairs in the form of:

``` ruby
{ :symbol_key => 'a value' }
```

have been converted to the "sexy syntax" introduced in Ruby since version 1.9,
that is:

``` ruby
{ symbol_key: 'a value' }
```

I think it's cleaner and it makes code more readable while not affecting it in
any other way. It's not a performance effort in any way.

Since Ruby isn't able to distinguish between old and new syntaxes, this commit shouldn't introduce bugs of any sorts; `rake test` gives no errors.