Bump version to 2.0.0rc6
Bump version to 2.0.0rc6

- Bumped version to future 2.0.0rc6 version
- Updated README to refer to 2.0.0rc6
- Added note in README about change in Zend\Mvc\Application EM instance
  identifiers

Maks3w/zend-version,MadCat34/zend-version,texdc/zend-version,remicollet/zend-version