Update gitignore file to ignore `vendor` folder changes

[whilei]

This is a controversial issue; whether or not to add vendor files to VC. Here's a discussion for example: https://www.reddit.com/r/golang/comments/8s1cjd/do_i_gitignore_the_vendor_folder_dep/

[whilei]

I guess personally I think I prefer to keep vendor in VC for always reproducible builds and simplified build steps (no need to give instructions for dep ensure).

If the pain for your PR is because it's based on commits prior to some vendor changes, have you tried

$ rm -rf ./vendor ./Gopkg* # nuke some vendoring things if you're feeling aggressive ;) but not necessary
$ git checkout master -- vendor Gopkg*
$ git clean -fd
$ git add . && git commit -m "Fake rebase vendor dir to master"

[r8d8]

Yeap, I can avoid vendor with removing it before commit. But as we using dep ensure in our Makefile, I thought it would be more transparent just to store project sources in git and download dependency during builds.

[tzdybal]

In my opinion we shouldn't add vendor to .gitignore. It's a 'traditional', pre-dep, dependency-tool agnostic way of vendoring dependencies in Go.

[whilei]

Marking as resolved now for the time being - we’ll leave vendor as-is.