Compatiblity problem between gradle-jpi-plugin:0.26.0 (latest) and gradle:<4.4
It report an exception `Unable to load class 'org.gradle.api.file.DirectoryProperty'` when I update the gradle plugin [gradle-jpi-plugin](https://github.com/jenkinsci/gradle-jpi-plugin) to version 0.26.0, which is the [latest release](https://github.com/jenkinsci/gradle-jpi-plugin/releases/tag/0.26.0) on 5 Apr 2018.
And it runs normally with `gradle-jpi-plugin:0.25.0`.
I figure out that it seems like this [commit](https://github.com/jenkinsci/gradle-jpi-plugin/commit/64ef3fd963080230c94c85beb1af0f07ab6b2aa7) break the compatibility.
In the commit, it import the class `org.gradle.api.file.DirectoryProperty`, which first occurs in gradle of version 4.4.
I think either adding a requirement for gradle version on README, or fixing this break should be done.