Fix bug: Version string 5.1 and 5.1.0 are now eql

Fix a bug that caused versions that had added zeroes and versions which
had no added zeroes to return false for `eql?`.
This was inconsistent with the operators <=, >= and <=> which all returned
equal.

Before:

    UserAgent::Version.new("6.1") == UserAgent::Version.new("6.1.0")
     => false 
    UserAgent::Version.new("6.1") <=> UserAgent::Version.new("6.1.0")
     => 0 

After:

    UserAgent::Version.new("6.1") == UserAgent::Version.new("6.1.0")
     => true 
    UserAgent::Version.new("6.1") <=> UserAgent::Version.new("6.1.0")
     => 0