Coverage should combine reports from all runs

Coveralls currently only runs on node 4.6; instead, we should get coverage on every test run, and after_success should istanbul-merge it, and send that to coveralls.

[ljharb]

@JaKXz A more specific example for this repo: if we have code that’s specific to React 15, and only react 15, we’d want the if branch (which only works on the React 15 job) and the else branch (only on the React 14 job) to both be counted for coverage.

[ljharb]

Absolutely there should; because different node versions can traverse different paths; let’s say if we’re conditionally checking the environment.

Proper coverage always requires checking and combining across multiple environments.