Implement GET /exchange-rate/<from>/<to>

Overview:
Implement the exchange-rate endpoint: find the exchange rate for: from -> to (both currency codes) and cache it.

Approach:

• Call the following API: https://api.exchangeratesapi.io/latest?base=<to>&symbols=<from>
• Pull out the exchange rate
• Cache the value
• Invalidate the cache every day at 1600 CET
• If the service is unavailable or errors occur during retrieval, use the last known value.
• If there is no last known value, use 1.0

Example Request/Response to Public API:

$ curl "https://api.exchangeratesapi.io/latest?base=USD&symbols=EUR" -w '\n' | jq .
{
  "base": "USD",
  "rates": {
    "EUR": 0.8929368694
  },
  "date": "2019-05-06"
}

Example Response to Client:

{
  "to": "USD",
  "from": "EUR",
  "rate": 0.8929368694,
  "date": "2019-05-06"
}

Acceptance Criteria:

• The latest daily exchange rate is obtained and cached as described above
• Unit tests have been updated
• API tests have been updated