feat: upgrade to latest istanbul api

[sebastianhaeni]

The package istanbul-api is deprecated.
See https://github.com/istanbuljs-archived-repos/istanbul-api

I migrated the code to use the new packages.
This brings in new underlying features of the istanbuljs library that I wish to use.

For example lcov has been changed to use relative paths and made the project root configurable.

[sebastianhaeni]

I'm guessing the build failed because Node.js 6 is no longer supported by the new packages that I installed. Istanbuljs itself only runs their CI tests from Node.js 8 and upwards.
See https://travis-ci.org/github/istanbuljs/istanbuljs

I'm also removing tests on Travis for Node.js 6 in this PR then.

[mattlewis92]

Hey, thank you so much for doing this migration, I really appreciate it! I left a couple of small comments. Also would it be possible for you to update the readme with the links to the latest reporter options? Thanks! 😄

[sebastianhaeni]

Thanks for the review! I updated the commits with the suggested changes.
Please have a look again.

[mattlewis92]

Perfect, thank you! 😄

[mattlewis92]

Released as 3.0.0, thanks for your contribution! 🎉

[XhmikosR]

Congrats on this, so happy to see such a big size decrease!

https://packagephobia.now.sh/result?p=karma-coverage-istanbul-reporter