Prevent branch builds publishing new gem versions

Version bumps now happen on branches, so this is required
to prevent them triggering a new gem version being published.

See: https://github.com/alphagov/gds-api-adapters/commit/003674e4cd7e9e93176cbe9be2e0717656678add
Merge branch builds against master before running tests

See https://github.com/alphagov/rummager/commit/698c2c896f9aa227dbd7dc8600c043008f084db7
Merge pull request #82 from alphagov/branch-builds-no-gem

Safer branch builds