Do not call update() in constructor

[fabaff]

Description:

Do not call update() in constructor.

Example entry for configuration.yaml (if applicable):

binary_sensor:
  - platform: arest
    resource: http://192.168.1.109
    pin: 4

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