Raise min versions of MW and PHP; clean-up CI testing config

[s7eph4n]

This PR is made in reference to: #36

This PR addresses or contains:

• New minimum required versions:
  • PHP 5.5
  • MediaWiki 1.27
• Fix some tests to avoid warnings from PHP 7.2

This PR includes:

• [x] Update of RELEASE-NOTES.md
• [x] Tests (unit/integration)
• [x] CI build passed