Fix Go client generation

[mraerino]

Closes #170

Summary

Updates to Go 1.13

Uses a workaround to ignore tags when generating the Go client.
Generating the Go client while the operations have different tags attached means generating a different package for every tag. This breaks the API for all users of the client because they will most likely rely on types for request params even if they are just using the porcelain package.

I opened #199 to track this issue.

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[ehmicky]

Thanks a lot for this @mraerino!

Three questions:

• are we always sure the Makefile will be executed on a machine that has sh (e.g. some Windows machines don't) and jq installed?
• we are already both validating and "flattening" the swagger.yml and converting it to swagger.json as part of npm run convert. This could be used instead of go-swagger validate and flatten methods?
• likewise we could remove the tags properties from that convert.js method, and output it to a second file swagger_go.json?

[mraerino]

Those are valid remarks. Can we move this to #199?