Stop testing against Ruby 1.8

- The only people this might affect are people running 1.8.7
- If people are running 1.8.7 they haven't updated this gem in 6 months (when we started tracking versions)
- If they haven't updated this gem in 6 months, it's unlikely we need to worry about them being desperate to upgrade now and providing backwards compatible support

@hmarr For now this just stops testing against old versions, once merged I'll rebase https://github.com/gocardless/gocardless-ruby/pull/70 and get green specs.

Sound good?