Rework change log for PHPMD 2.7.0
Rework change log for PHPMD 2.7.0

- Add reminder for BC breaking change in release intro
- Add PHP bump in release intro
- Reorder changes to (more or less) reflect their importance (new features first, then improvements & fixes, later dependency & build changes, style & documentation last)
- Remove duplicated entries
- Improve wording of entries

phpmd/phpmd,ravage84/phpmd