Change deprecated classesDir for classesDirs

[pablisco]

Note: copy of #170 due to rebase

classesDir is deprecated in Gradle 4.x and we should use classesDirs instead as Gradle 5.x removes this property.

Without this change, Gradle complains about it if we use Gradle 5.x on the client:

* What went wrong:
Could not determine the dependencies of task ':core-java:findbugsMain'.
> Could not get unknown property 'classesDir' for main classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput.

Fixes #94

[tasomaniac]

@hal90002 retest this please

[tasomaniac]

Couldn't flush user prefs: java.util.prefs.BackingStoreException: Couldn't get file lock.

[pablisco]

Oh dear... 🙃

org.gradle.tooling.GradleConnectionException: Could not execute build using Gradle installation '/storage/jenkins/.gradle/wrapper/dists/gradle-4.10.2-all/9fahxiiecdb76a5g3aw9oi8rv/gradle-4.10.2'.

[tasomaniac]

Somebody restart the machine.

[mr-archano]

@hal90002 retest this please

[mr-archano]

FYI @pablisco the issue with this build should be related to the presence of emojis in the PR description. I can't remember exactly why but Jenkins fails to parse that string correctly and crashes.

I have removed the 🙃 from the description and re-triggered the build, and now I would expect the CI to go ✅

[tasomaniac]

Come on seriously. 😂 Not again.

[tasomaniac]

Running for almost 2 hours. It used to be around 30min

[mr-archano]

@tasomaniac there must be some issue with the CI. I run a test build from develop (#173) and it's the same issue :/

[mr-archano]

@hal90002 retest this please 🤞🤞🤞

[mr-archano]

@pablisco I believe this PR is unrecoverable for the CI, needs to be re-open. I run the tests on my machine and they seem fine. Could you please issue the PR again (without emoji in the description 😅)

[tasomaniac]

What about manually merging?

Btw, hour long did they take locally?