Release v2.0

Adjust packages version

Adjust packages version to release 2.0 stable.
This commit closes #51.

Release v2.0-alpha2

Adjust versions

Adjust versions of the required packages, so that ther libraries can
be installed separately, also in pre-release versions.

Version 1.2

• Changed: Minimum PHP version 5.6
• Updated dependencies
• Removed: Dropped code for php < 5.6

Version 1.1

• Added Json::toCollection()

Version 1.0

First version