Assignment for 888

1. App port is 8081
2. Cache keys have expiration period 5 minutes
3. Continents codes are case sensitive
4. There preparation for support getting data from non-GraphQl sources (for example REST)
5. Basic handling of errors : wrong routes, invlid continent code
6. NOT handled redis diconnection

There were some issue when running via docker. In order to run without docker , in file

cache-helper.js

const client = redis.createClient ({host: 'redis-server', port: 6379});

by

const client = redis.createClient ({port: 6379});

added test str