Outdated version of golangci-lint doesn't work with current version of go.

[elsiehupp]

I ran into this issue when trying to build podsync as-is on my computer (see #100), and it only compiles with go 1.13.6.

The error seems to be coming from the Makefile calling an outdated version of golangci-lint, as the build validates if I comment out the references to linting.

I temporarily forked podsync in order to work on the feed metadata, and I'm running into the same issue there: https://github.com/elsiehupp/podsync

I've never worked on an open-source project before, and I'm not sure how pull requests work, but I would like to be able to reintegrate the changes I've made as soon as possible.