Rename Rake task for integration tests

Rename the Rake task for integration tests from 'integration_test' to
'integration' to keep it consistent with other tools[1][2][3].

[1]:
https://github.com/alphagov/vcloud-net_launcher/blob/master/Rakefile#L20
[2]: https://github.com/alphagov/vcloud-launcher/blob/master/Rakefile#L6
[3]:
https://github.com/alphagov/vcloud-edge_gateway/blob/master/Rakefile#L15