Moved VERSION into a single file

You can now update VERSION via just one file:
lib/net/ldap/version.rb

This makes maintenance easier.

PS:

I'm rather confused about if this is the official net-ldap github repository or not:

* [RubyGems](http://rubygems.org/gems/net-ldap) lists [Rory-O/ruby-netldap](https://github.com/RoryO/ruby-net-ldap) as the location of the source.
* [RubyForge](http://net-ldap.rubyforge.org/) points here, but is version 0.2.2
* [netldap.org](https://github.com/ruby-ldap/ruby-net-ldap) points here.

To add to the confusion, I don't see version 0.3.1 in this repository but maybe I missed it?