Fix deprecation warning `The DefaultSourceDirectorySet constructor ha…

…s been deprecated`.

- Fixed by using the ObjectFactory service to create the source directory set

This fixes the following deprecation warning seen in any projects that apply the gradle-css-plugin:

```
The DefaultSourceDirectorySet constructor has been deprecated.
This is scheduled to be removed in Gradle 6.0.
Please use the ObjectFactory service to create instances of SourceDirectorySet instead.
```

src/main/groovy/com/eriwen/gradle/css/source/internal/DefaultCssSourceSet.groovy

@@ -24,7 +24,9 @@ class DefaultCssSourceSet implements CssSourceSet {
     DefaultCssSourceSet(String name, Project project, Instantiator instantiator, FileResolver fileResolver) {
         this.name = name
         this.displayName = GUtil.toWords(name)
-        if (GradleVersion.current().compareTo(GradleVersion.version("2.12")) >= 0) {
+        if (GradleVersion.current().compareTo(GradleVersion.version("5.0")) >= 0) {
+            this.css = project.objects.sourceDirectorySet(name, String.format("%s CSS source", displayName))
+        } else if (GradleVersion.current().compareTo(GradleVersion.version("2.12")) >= 0) {
             Class fileTreeFactory = Class.forName("org.gradle.api.internal.file.collections.DefaultDirectoryFileTreeFactory")
             def directoryFileTreeFactory = fileTreeFactory.getConstructor().newInstance()
             this.css = new DefaultSourceDirectorySet(name, String.format("%s CSS source", displayName), fileResolver, directoryFileTreeFactory)