Use nyc instead of babel-istanbul for code coverage

[rewop]

For better support of babel, and for simpler setup, we are now using https://github.com/istanbuljs/nyc in several of our services.

We could update this boilerplate so it will be set already for future modules.

How to set it up:

• update package.json scripts with the following lines

"mocha": "NODE_ENV=test mocha --timeout 10000 --compilers babel-register --recursive $(find src -name __test__ -not -path '*/node_modules/*')",
    "test": "NODE_ENV=test nyc --reporter=lcov --reporter=text-summary --reporter=html npm run mocha",
    "cov-check": "nyc check-coverage --statements 80 --branches 70 --functions 60",
    "cov-report-html": "open coverage/index.html",
    "cov-report-text": "nyc report"

• remove babel-istanbul dependency
• install dev dependencies nyc and babel-plugin-istanbul
• Add istanbul plugin in .baberc file