Fix deprecation warning "The DefaultSourceDirectorySet constructor has been deprecated"

[robinverduijn]

Fix deprecation warning The DefaultSourceDirectorySet constructor has 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.

There is an additional Gradle deprecation warning in this project, but that already has a PR open for it at https://github.com/eriwen/gradle-css-plugin/pull/52 so I am not duplicating those changes here.

[edgargs]

@eriwen This is a import fix.

[robinverduijn]

I only fixed the other remaining deprecation warning, but you would also need to get the fixes from an as-yet-unmerged pull request at eriwen#52 to be completely warning-free, I think.

It looks like by now, the changes from https://github.com/eriwen/gradle-css-plugin/pull/52 should no longer be necessary as https://github.com/eriwen/gradle-css-plugin/commit/9fe88d712636a9c81f877d23d531bddd285dfc0b has dropped that class entirely. I have rebased my branch to pull in the latest master and I think these are now the only remaining changes needed to fix the warnings seen under the latest Gradle versions.

I also pushed a change to fix the Travis CI build by updating the build container to oraclejdk11.

[robinverduijn]

@eriwen any concerns with this pull request? Could it be merged?