Stringify NRPE port in VSE rule

To fix schema validation error:

    E, [2014-05-09T09:54:01.995771 #6275] ERROR -- : Supplied configuration does not match supplied schema
    F, [2014-05-09T09:54:01.995619 #6275] FATAL -- : destination_port_range: 5666 is not a string
Merge pull request #47 from alphagov/stringify_vse_rule_port

Stringify NRPE port in VSE rule