Update protoc and related dependencies

[rvolosatovs]

Summary

Regular dependency update

Changes

• Update protoc and related dependencies

Notes for Reviewers

Seems like some validation rules were missed before, which are now generated. Also some Swagger changes.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md. The target branch is set to master if the changes are fully compatible with existing API, database, configuration and CLI.
• Testing: The changes are covered with unit tests. The changes are tested manually as well.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[johanstokking]

LGTM

[rvolosatovs]

@htdvisser I attempted to update protobuf to 1.4.0, but that caused pkg/errors/web test failure, which I was not sure how to fix(and not sure whether it represents a regression somewhere), hence reverting the change and letting you handle this in next dependency update PR:

--- FAIL: TestErrorHandling (0.00s)

    --- FAIL: TestErrorHandling/Echo404 (0.00s)

        doc.go:66: 

            /home/travis/gopath/src/go.thethings.network/lorawan-stack/pkg/errors/web/middleware_test.go:46

            Expected '{"code":5,"message":"error:pkg/errors/web:unknown (Not Found)","details":[{"type_url":"type.googleapis.com/ttn.lorawan.v3.ErrorDetails","value":"Cg5wa2cvZXJyb3JzL3dlYhoJTm90IEZvdW5kIhgKFgoHbWVzc2FnZRILGglOb3QgRm91bmQ4BQ=="}]}

            ' to contain substring '"namespace":"pkg/errors/web"' (but it didn't)!

FAIL

coverage: 66.7% of statements

FAIL	go.thethings.network/lorawan-stack/pkg/errors/web	0.030s

[coveralls]

Coverage increased (+0.005%) to 73.003% when pulling 37ecf0e on rvolosatovs:feature/update-protoc into ad0fd12 on TheThingsNetwork:master.