Make SOPS dependency optional

[pierec]

Allow to install Oya on environments without SOPS. Some projects might not want to use the "secrets" feature and should not be forced into installing unnecessary dependencies.

Currently attempting to install Oya on a SOPS-free env gives the following error message:

Run curl https://oya.sh/get | bash
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
Checking required tools:
 Sops -> FAIL
                                 Dload  Upload   Total   Spent    Left  Speed

Oya requires SOPS (Secrets OPerationS) to manage secret files.
Visit https://github.com/mozilla/sops for install instructions.