chore(coverage): rollback upgrade to istanbul that caused build breakage
chore(coverage): rollback upgrade to istanbul that caused build breakage

related #646

elliotschi/rxjs,figueredo/RxJS,adrianomelo/RxJSNext,ReactiveX/RxJS,martinsik/rxjs,martinsik/rxjs,Brocco/RxJS,Brocco/RxJS,SangKa/RxJS-Docs-CN,jinroh/RxJS,IgorMinar/RxJS,adrianomelo/RxJS-1,Brocco/RxJS,adrianomelo/RxJSNext,saneyuki/RxJS,jayphelps/rxjs,jinroh/RxJS,jayphelps/rxjs,trxcllnt/RxJS,saneyuki/RxJS,jadbox/RxJS,gluck/RxJS,ReactiveX/rxjs,jinroh/RxJS,kwonoj/RxJS,ReactiveX/rxjs,mattpodwysocki/RxJS,adrianomelo/RxJS-1,elliotschi/rxjs,SangKa/RxJS-Docs-CN,jayphelps/rxjs,martinsik/rxjs,robwormald/RxJS,Brocco/RxJS,jinroh/RxJS,justinwoo/RxJS,ReactiveX/RxJS,justinwoo/RxJS,gluck/RxJS,benlesh/RxJS,adrianomelo/RxJS-1,benlesh/RxJS,adrianomelo/RxJSNext,trxcllnt/RxJS,benlesh/RxJS,jayphelps/rxjs,IgorMinar/RxJS,robwormald/RxJS,jadbox/RxJS,martinsik/rxjs,trxcllnt/RxJS,RxJS-CN/RxJS-Docs-CN,kwonoj/RxJS,SangKa/RxJS-Docs-CN,benlesh/RxJS,staltz/RxJSNext,Igorbek/rxjs5,Igorbek/rxjs5,kwonoj/RxJS,Igorbek/rxjs5,robwormald/RxJS,ReactiveX/RxJS,ReactiveX/RxJS,mattpodwysocki/RxJS,Igorbek/rxjs5,staltz/RxJSNext,SangKa/RxJS-Docs-CN,RxJS-CN/RxJS-Docs-CN,figueredo/RxJS,jadbox/RxJS,justinwoo/RxJS,RxJS-CN/RxJS-Docs-CN,elliotschi/rxjs,trxcllnt/RxJS,mattpodwysocki/RxJS,adrianomelo/RxJSNext,gluck/RxJS,ReactiveX/rxjs,gluck/RxJS,figueredo/RxJS,elliotschi/rxjs,figueredo/RxJS,mattpodwysocki/RxJS,jayphelps/rxjs,robwormald/RxJS,kwonoj/RxJS,staltz/RxJSNext,justinwoo/RxJS,RxJS-CN/RxJS-Docs-CN,staltz/RxJSNext,jadbox/RxJS,adrianomelo/RxJS-1,ReactiveX/rxjs