Detect version based on tag if HEAD points to a tag.
Fixed the existing test and added a new one for git tag version guessing.
Use --tags to get non-annotated tags as well.
Merge remote-tracking branch 'simensen/non-dev-version'

Conflicts:
	src/Composer/Package/Loader/RootPackageLoader.php
	tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php