go.mod: update dependencies and tidy up

[fsouza]

This is the result of updating aws-sdk-go and running go mod tidy. A lot
has happened since go modules where introduced in this repo, so go mod
tidy refreshes a lot of things.

The motivation is that we use this as a library in gizmo and we want to make sure our deps are fresh :)

[odeke-em]

Awesome, thank you @fsouza for this tidying up #punUnintended
and great to hear that you and your team at New York Times are using OpenCensus.

LGTM, but I have some suggestions:
a) Have you taken a look at the new OpenCensus Agent https://opencensus.io/agent/?
With the agent, you'll no longer have to add each exporter in your code. You'll just need to enable the
ocagent-go-exporter and then running your agent out of process configure the agent to export to a variety of backends https://opencensus.io/agent/exporters/
b) If you and your team are comfortable publicly declaring yourselves OpenCensus users, would you mind sending a PR to https://github.com/census-instrumentation/opencensus-website adding yourselves as users to https://opencensus.io/community/users/
If you'd like me to do so, and y'all provide the approval I'd be delighted to do so :)

[odeke-em]

@songy23 @rghetia let's also publish a minor release so that folks can have these cleaned up dependencies.

[rghetia]

sure.

[fsouza]

Hey @odeke-em, I wasn't aware of the agent, I'll share with folks internally and see if there's interest. I suppose we may want to keep it as is, so we don't have to run the agent 😁 (even if we do it as a side-car, it's still one more thing to run).

Regarding putting the NYT logo in the users section, that can be tricky :(((