Stablize omnitools library version

Summary:
The Keruald OmniTool library at 0.2.0 version contains all the functions
previously contained in this library.

As such, this becomes the minimal version to require. As OmniTool adheres
to semantic versionning, there is a choice between ~0.2 (0.2.*) and ^0.2 (0.*).

As 0.3 will be used to add new functions more than refactor the existing ones,
the caret operator seems more convenient.

Ref T1395.

Test Plan: composer validate

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1395

Differential Revision: https://devcentral.nasqueron.org/D1626