Various improvements.

Please let me know if you require any changes.

Thanks for the plugin.
See https://github.com/ratpack/ratpack/blob/master/ratpack.gradle#L299

This is a common pattern in Gradle to allow the use of a File object if you have one or a string if you want to use a project relative path.

See the corresponding use of [`Project.file()`](http://www.gradle.org/docs/current/dsl/org.gradle.api.Project.html#org.gradle.api.Project:file\(java.lang.Object\)) method [here](https://github.com/ratpack/coveralls-gradle-plugin/commit/b93aefa6558ca54eec51662e5b8929ff970f17c5#diff-bd91f163995c3bdc862bc506700c4a96R92)