….6-release

Readme: minor tweak now 0.6.0 has been released

If the phpcodesniffer-composer-installer plugin is installed as a dev
requirement and it is then uninstalled as part of a "--no-dev" install, a bug
occurs. The bug that occurs is that the plugin complains that the  package
"squizlabs/php_codesniffer" is not installed without checking if the package
should be present.

This commit adds a check to verify that this plugin is actually installed before
complaining about the missing package. If this plugin itself is removed, then
it should not complain about the missing package.

Previously the check would only run if the plugin was require-dev.
That behaviour is incorrect, as the check should always be done.
If the plugin is not installed, it doesn't make sense to return an
error under any circumstances.

This commit fixes that oversight.

…unistall

Fix for issue #103