Added YAML and PDO extensions to dev dependencies

YAML and PDO are optional dependencies.

Users of the package must install them themselves and may specify them as requirements in their `composer.json` file (if they are using the YAML or the database adapter).

However for running the testsuite of the package they are required. So they should be specified as development dependencies.
Merge pull request #1 from hkdobrev/patch-1

Added YAML and PDO extensions to dev dependencies