Correct return type for Redis.quit()

[dex4er]

Fixes:

test/lib/mock-ioredis.ts:48:3 - error TS2416: Property 'quit' in type 'MockIORedis' is not assignable to the same property in base type 'Redis'.
  Type '() => Promise<string>' is not assignable to type '{ (callback: Callback<"OK">): void; (): Promise<"OK">; }'.
    Type 'Promise<string>' is not assignable to type 'Promise<"OK">'.
      Type 'string' is not assignable to type '"OK"'.

48   quit(): Promise<string> {
     ~~~~

[coveralls]

Coverage increased (+0.3%) to 89.13% when pulling c32d62c on bugfix-mock-ioredis into 534019c on master.