Verify vendored modules in pull requests

[eloyekunle]

What would you like to be added

Add scripts to update vendor and verify vendor. Vendor verification should run in Travis for all pull requests to ensure the go.mod file includes the correct dependencies.
The update vendor script will run go mod tidy to prune all extraneous requirements.

Why is this needed

To ensure that the contents of the go.mod file and the /vendor directory are always correct and verified.

Comments

This is based on the current implementation in the Kubernetes repository:

1. The update-vendor.sh script.
2. The verify-vendor.sh script.

[eloyekunle]

Closing. This is unnecessary since we're not keeping the vendor directory anymore. Tidying and bumping packages can be done manually.