tests: add travis-ci testing config for app-container

This adds a .travis.yml file to run the app-container tests, although it points out currently that one of them needs to be upated:
```console
$ go test -v ./app-container/...
# github.com/coreos/rocket/app-container/discovery
app-container/discovery/discovery_test.go:33: too many arguments in call to DiscoverEndpoints
```

I didn't include go 1.2 in the matrix because it has a separate error during the `install` task, and I wasn't sure if 1.2 support was required:
```console
$ go get -d -v ./app-container/... && go build -v ./app-container/...
github.com/coreos/rocket (download)
# github.com/mboersma/rocket/app-container/ace
app-container/ace/validator.go:214: unknown http.Client field 'Timeout' in struct literal
```
Refs #169. Requires the Travis CI webhook to be enabled, either in GitHub's project settings, or by toggling it on in Travis CI's settings page (easier, IMHO).