Bump travis-build once and for all

It's now an API!
Pull build script from API
runner: don't merge in config data

The Script class merges this in already, so no need doing it twice.
script: send a user-agent to the script API
script: add a timeout
remove leftover travis-build require
script: JSON encode the payload
api: set the Accept header

This allows us to more flexibly change what the API does by switching on
the Accept header in the future.
Merge remote-tracking branch 'origin/master' into hh-build-api
script: fix typo 'etch_hosts' -> 'etc_hosts'
script: use travis-build locally if the API config isn't set

This allows us to deploy the code to all workers and then gradually
switch over to using the API.
Merge branch 'master' into hh-build-api

Conflicts:
	Gemfile.lock
Merge branch 'master' into hh-build-api
Merge branch 'master' into hh-build-api
Merge branch 'master' into hh-build-api
script generation: add logging
script generation: add metrics
Fix Job::Script spec

The constructor added a new parameter, so we need to pass in the
parameter in the specs.
Script generation: Require 'metriks'

This is already required elsewhere in the code, but needs to be required
here too for the specs to pass (since the specs only requires this
library).

I also think it's good form to require all the dependencies for a few
reasons:

  - Makes it easy to see what this file depends on (and a red flag can
    be raised if there are too many).
  - If the other file ever stops requiring the dependency, this file
    could break as it is no longer loaded.
Merge pull request #94 from travis-ci/hh-build-api

Use build API