Add ci:last command

Handles when there is a last run and when there is not. It depends on
the call to test runs to return an ordered array of runs, created_at
desc.

commands/ci/last.js

@@ -0,0 +1,28 @@
+const cli = require('heroku-cli-util')
+const co = require('co')
+const api = require('../../lib/heroku-api')
+const TestRun = require('../../lib/test-run')
+
+function* run (context, heroku) {
+  const coupling = yield api.pipelineCoupling(heroku, context.app)
+  const pipeline = coupling.pipeline
+  const pipelineID = pipeline.id
+  const runs = yield api.testRuns(heroku, pipelineID)
+
+  if (!runs[0]) {
+    return cli.error('No Heroku CI runs found for this pipeline.')
+  }
+
+  const runNum = runs[0]['number']
+  return yield TestRun.displayAndExit(pipeline, runNum, { heroku })
+}
+
+module.exports = {
+  topic: 'ci',
+  command: 'last',
+  needsApp: true,
+  needsAuth: true,
+  description: 'get the results of the last run',
+  help: 'looks for the most recent run and returns the output of that run',
+  run: cli.command(co.wrap(run))
+}