Error when pruning is skipped

[micahjsmith]

When a user has disabled the pruning job, the bot has an error:

TypeError: Cannot read property '2' of undefined
  File "/app/lib/travis.js", line 73, in getJobLogFromBuild
    const prunerJob = build.jobs[jobNum];
  File "/app/lib/travis.js", line 35, in Object.getTravisAcceptanceMetadata
    const acceptanceLog = await getJobLogFromBuild(
  File "/app/index.js", line 72, in null.
    const { omittedFeature } = travis.getTravisAcceptanceMetadata(travisBuildId);

Suggested behavior:

• the bot checks the job exists before trying to get the job log