Builder proceeds if slugrunner pod is evicted

My slugrunner pod is quite often evicted due to low compute resources on the node.

An example log of a git push of a buildpack build.

```
[chexxor@fedora myapp]$ git push ssh://git@deis-builder.123.456.789.012.nip.io:2222/myapp-master.git master
Counting objects: 4129, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3757/3757), done.
Writing objects: 100% (4129/4129), 5.39 MiB | 4.75 MiB/s, done.
Total 4129 (delta 2821), reused 474 (delta 288)
remote: Resolving deltas: 100% (2821/2821), done.
Starting build... but first, coffee!
-----> Restoring cache...
Done!
-----> Node.js app detected

-----> Creating runtime environment

NPM_CONFIG_LOGLEVEL=error
NPM_CONFIG_PRODUCTION=true
NODE_ENV=production
NODE_MODULES_CACHE=false

-----> Installing binaries
engines.node (package.json): 4.6.0
engines.npm (package.json): 2.15.x

Downloading and installing node 4.6.0...
Resolving npm version 2.15.x via semver.io...
Downloading and installing npm 2.15.11 (replacing version 2.15.9)...

-----> Restoring cache
Skipping cache restore (disabled by config)

-----> Building dependencies
Running heroku-prebuild

> rentable@1.0.0 heroku-prebuild /tmp/build
> echo "Prebuild steps running..."

Installing node modules (package.json)
Build complete.
Launching App...
...
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...ote:
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...ote:
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Done, myapp-master:v76 deployed to Workflow
```

Note that the slugbuilder pod was evicted while executing the `-----> Building dependencies` step. I believe this because the logs produced by this step *should be* hundreds of lines, and the following buildpack steps don't appear, like ["-----> Caching build"](https://github.com/heroku/heroku-buildpack-nodejs/blob/master/bin/compile#L169) and ["-----> Build succeeded!"](https://github.com/heroku/heroku-buildpack-nodejs/blob/master/bin/compile#L178).

Despite this slugbuilder pod failing, the builder process continues, and prints ["Build complete.](https://github.com/deis/builder/blob/4634fe14784c954027af167d15ff8d3490cf4942/pkg/gitreceive/build.go#L306) skipping the [failed pod check](https://github.com/deis/builder/blob/4634fe14784c954027af167d15ff8d3490cf4942/pkg/gitreceive/build.go#L293-L304).

I upgraded my workflow just a few days ago, so I believe I have the latest versions of these components.