Pin `tins` dependency to ruby 1.9.3-compatible version
Pin `tins` dependency to ruby 1.9.3-compatible version

`tins` is a dependency of `term-ansicolor`, which is a dependency of
`pact.

A recent change in `tins`
(https://github.com/flori/tins/commit/9ebb625d8a987bd54526006441ff97bc14
c30e53) made ruby 2.0.0 required for that gem.

Because we pull in the latest version and `term-ansicolor` allows
upgrades to any `1.X`, our tests have started failing after `tins`
published 1.7.

alphagov/gds-api-adapters