Fix Mocha warning about --compilers (#3183)
Fix Mocha warning about --compilers (#3183)

See https://github.com/mochajs/mocha/wiki/compilers-deprecation
Joe-noh/phoenix,0x00evil/phoenix,nshafer/phoenix,nshafer/phoenix,keathley/phoenix,jwarwick/phoenix,optikfluffel/phoenix,dmarkow/phoenix,Gazler/phoenix,0x00evil/phoenix,evax/phoenix,0x00evil/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,eksperimental/phoenix,eksperimental/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,mitchellhenke/phoenix,phoenixframework/phoenix,Gazler/phoenix,jwarwick/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,nshafer/phoenix,evax/phoenix,dmarkow/phoenix,rafbgarcia/phoenix,Joe-noh/phoenix,keathley/phoenix,mitchellhenke/phoenix