Add a .ruby-version file

To ensure a consistent build on the CI environment
Add a .travis.yml with matrix build
Updates to allow support for ruby 1.8.7

There were only a couple of things preventing this from working with
1.8.7:

- the 1.9 hash syntax was used in a couple of places
- 1.8.7 doesn't include minitest - fixed by adding as a gem dependency
Merge pull request #16 from alphagov/ruby_1.8.7

Updates to allow support for ruby 1.8.7