disable coveralls on forked PR builds
disable coveralls on forked PR builds

see https://circleci.com/docs/2.0/env-vars/#circleci-environment-variable-descriptions
for variable descriptions

compare https://circleci.com/gh/StoDevX/AAO-React-Native/8051
and https://circleci.com/gh/StoDevX/AAO-React-Native/8017

to check that CIRCLE_PR_NUMBER is only available on forked PR builds

carls-app/carls,carls-app/carls,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,carls-app/carls,carls-app/carls,StoDevX/AAO-React-Native,carls-app/carls,StoDevX/AAO-React-Native,carls-app/carls,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,carls-app/carls,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native