Fix failing tests

[michaelsunglee]

This PR fixes failing tests for Kunaki, USPS, and Correios. Kunaki

• Change the URL scheme
• Replace old SKUs with new test SKUs provided by Kunaki

USPS

• Removed the outdated test test_tracking_with_attempted_delivery. This test used a live tracking number which is now out of use and thus returned an error. Adding a new "live" tracking number is only temporary and isn't a solution. USPS does not provide any testing tracking numbers either.

Correios

• It seems that this carrier has stopped supporting their API endpoints. All sources point to this sign. For example this site refers to this simple zip lookup which fails. Other open source wrappers for this API have either reported this problem or have been archived themselves (for example). I've tested a few other wrappers and they all seem to fail. For these reasons I think Correios should not be supported (at least for now) in active_shipping as the failing tests are slowing development.

This PR also fixes the Travis CI build errors around activesupport (it required Ruby version >= 2.4.1). The build also supports activesupport 6.0.0 and so the dependencies reflect that (as well as creating activesupport51.gemfile for more test builds).

Please review changes. @rafaelfranca @Edouard-chin @tamcgoey @garethson

[rafaelfranca]

Correios

As a Brazilian it makes me sad that correios broke their API 😢