Support Go 1.14 (#419)

* Upgrade base image tags and golangci-lint

* Upgrade golang and golangci-lint in travis-ci

* fix:  tests, pointed out by golangci-lint and format files

* Upgrade dependencies

* update changelog

* fix: tests related to https://github.com/pusher/oauth2_proxy/pull/418



* Separate tests using go version build tags

* Update CHANGELOG

* Revert "Separate tests using go version build tags"

This reverts commit 9b7e65eb90cae954dc7b6316345d3207205d488a.

* fix test to support go1.14 and go1.13

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>