Update Codecov Setup instructions to use single-line bash command

[nelsonic]

In the "Tracking Coverage as-a Service" section: https://github.com/dwyl/learn-istanbul/tree/bb027a62b40558ad1717fd8f168291777b95b8cd#tracking-coverage-as-a-service We are currently advising people to use the "old" way:

We can simplify this a lot by simply running the bash command:

bash <(curl -s https://codecov.io/bash)

relates to #12