From Istanbul to NYC

[s1na]

According to the comments in #327, this PR replaces istanbul with nyc for test coverage.

[s1na]

@holgerd77 Regarding the coveralls error, do you think something like this is worth giving a try?

The coveralls project has a secret repo token, which should be on the project page of their website. This is apparently required for circleci to work properly, specially in forked repos, or when running it locally.

I found three ways of providing this token to coveralls. Setting it as an environmental variable when running the command locally (COVERALLS_REPO_TOKEN=secret npm run coveralls), putting it in .coveralls.yml which isn't the best idea as it is secret, configuring it in the environment variables of CircleCI project setting.

I think taking the third approach would fix the PR issues for forks, which would be already great. However, I haven't tested it.

[holgerd77]

Looks good.

[holgerd77]

@s1na I actually did version 3, that's what puzzles me.