Migrate Build to Nebula

[benjchristensen]

Using plugin from https://github.com/nebula-plugins/gradle-rxjava-project-plugin

/cc @quidryan

[cloudbees-pull-request-builder]

RxJava-pull-requests #1541 FAILURE
Looks like there's a problem with this pull request

[benjchristensen]

Failed with:

> License violations were found: /scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/operators/TakeLastQueueProducer.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/SynchronizedSubscription.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/MessagePassingQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/MpmcArrayQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/ConcurrentCircularArrayQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/ConcurrentSequencedCircularArrayQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/SpmcArrayQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/SpscArrayQueue.java,/scratch/jenkins/workspace/RxJava-pull-requests/rxjava/src/main/java/rx/internal/util/unsafe/Pow2.java}

Here is an example file that is failing that shouldn't cause it to fail: https://github.com/ReactiveX/RxJava/blob/1.x/rxjava/src/main/java/rx/internal/util/unsafe/MessagePassingQueue.java

There are some legit ones where it is missing a license. I'll fix those.

[benjchristensen]

I fixed the one that was completely the missing. The rest have a header, but the validation is failing because it is different than it expects. They are all in: https://github.com/ReactiveX/RxJava/tree/1.x/rxjava/src/main/java/rx/internal/util/unsafe

They don't have a Copyright at the top and have comments about where the code originally came from.

/rxjava/src/main/java/rx/internal/util/unsafe/MessagePassingQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/MpmcArrayQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/ConcurrentCircularArrayQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/ConcurrentSequencedCircularArrayQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/SpmcArrayQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/SpscArrayQueue.java
/rxjava/src/main/java/rx/internal/util/unsafe/Pow2.java

These should not cause the build to break.

[benjchristensen]

Replaced with #1644