Remove golang 1.6.3 and 1.7
Remove golang 1.6.3 and 1.7

These versions are no longer used. 1.7.1 has been left to support
building router until it has been upgraded to 1.9.1
[PR](https://github.com/alphagov/router/pull/129)

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet