Do not call update() in constructor

[fabaff]

Description:

Do not call update() in constructor.

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: yahoo_finance
    symbols:
      - RHT
      - GOOGL
      - FAAFD
  - platform: hddtemp
    host: localhost

Checklist:

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @nkgilley, @colinodell and @ttroy50 to be potential reviewers.