Unable to resolve dependencies with dep on master

When running `dep ensure` on the master branch as of this writing (91b3b08f7dd666916039faafa9ce76c2540de852), `dep` is unable to find a solution and produces the attached output with the verbose flag turned on. This is using the latest version of dep built from master.

It appears that it tries all tagged revisions and the master branch of `github.com/goller/prometheus`, but never tries the `logger-targetmanager-wait` branch where it would (in theory) find a solution.

Dep output: [depout.txt](https://github.com/influxdata/kapacitor/files/1019378/depout.txt)

Reproduce the dependency-resolution failure with `dep ensure` on the Kapacitor master commit 91b3b08f7dd666916039faafa9ce76c2540de852, using the attached verbose `depout.txt` output as the starting evidence. Trace why the resolver does not consider github.com/goller/prometheus's `logger-targetmanager-wait` branch; done means dependency resolution finds a valid solution or the failure is clearly explained.

Written by the indexing model from the issue text.