Add TZInfo::Version

Please see [the discussion](https://github.com/tzinfo/tzinfo/issues/28) for more information.
Tested it locally and version is a proper constant.

````
1.9.3-p194 :001 > require 'tzinfo'
 => true
1.9.3-p194 :002 > TZInfo
 => TZInfo
1.9.3-p194 :003 > TZInfo::Version
 => "1.2.2"
````
P.S. removed some trailing space.