Create a vendor check for dependencies

[mandrigin]

Problem

Currently, our CI process doesn't ensure that the dependencies in vendor folder are the same as in Gopkg.lock file.

Implementation

The script should run dep ensure and then check if anything was changed in the vendor directory. If so — fail the pipeline.

The command to run this script should be make vendor-check

See this thread for more practical advice.

1st party implementation is coming to dep: golang/dep#121.

Acceptance Criteria

1. A makefile target is created that fails if there is something in a vendor folder that isn't in Gopkg.lock. Or if dep ensure changes something in the vendor folder.

2. .travis.yml is updated to have a step of checking dependencies before building or running any tests. (NOTE: Travis restarts builds on every push. If you need to force restarting it and have no changes to commit, just alter the latest commit message with git commit --amend and push. That should do the trick).

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