Use build script from payload if one is provided

This allows us to move the compilation of build scripts to hub, which
again means that updating travis-build doesn't require us to update all
the workers, allowing for quicker and more easy deployments of
travis-build.
Merge branch 'master' into bluebox-production-com
Merge branch 'bluebox-production-org' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Make Travis::Worker::Instance specs pass
Fix Config spec
Fix Factory spec
Fixed Buffer spec
Merge branch 'staging' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge branch 'master' into bluebox-production-com
Merge pull request #77 from travis-ci/hh-ah-fix-tests

Fixing tests
Merge pull request #72 from travis-ci/hh-get-script-from-payload

Use build script from payload if one is provided
Made decoded_payload a Hashr in instance_spec
Merge pull request #79 from Aaron1011/payload_hashr

Made decoded_payload a Hashr in instance_spec
Added build status image [ci skip]