Upgrade and unpin devDependencies (ethjs#16)

* remove lint:staged script
* devDeps: remove unused eventsource-polyfill
* devDeps: bn.js@4.11.6->^5.2.1
* devDeps: bignumber.js@3.0.1->^9.1.2
* devDeps: rimraf@2.3.4->^3.0.2
* devDeps: mocha@3.1.2->^7.2.0
* devDeps: chai@3.5.0->^4.3.10
* devDeps: pre-commit@1.1.3->^1.2.2
* devDeps: unpin all remaining
* npm dedupe; npm audit fix

Author: legobeat