Update to support NXRM 3.15

[nblair]

This change set updates to link against NXRM 3.15.2-01.

The new version of NXRM provides a number of new support classes to handle boiler plate for Blobstore implementations (see BlobStoreSupport, BlobStoreMetricsStoreSupport, BlobAttributesSupport, AttributesLocation).

Other notable changes:

• We have some new clarity on the contract Blobstore#getBlobIdStream. In previous versions, the Stream returned by this method in this plugin did not include temporary blobs. In this version, temporary BlobIds are included.

• A new integration test has been added (originally developed in Experiment: Parallel Multipart Upload #22).

• The travis CI build has been updated to use openjdk8.

• Fixes Support for 3.15 #24

[nblair]

Am seeing an entry in the log:

2019-03-02 04:20:41,876+0000 ERROR [blobstore-metrics-6-thread-1]  admin org.sonatype.nexus.blobstore.internal.PeriodicJobServiceImpl - Periodic job threw exception
java.lang.NullPointerException: null
	at org.sonatype.nexus.blobstore.quota.BlobStoreQuotaSupport.quotaCheckJob(BlobStoreQuotaSupport.java:97)
	at org.sonatype.nexus.blobstore.quota.BlobStoreQuotaSupport.lambda$0(BlobStoreQuotaSupport.java:84)
	at org.sonatype.nexus.blobstore.internal.PeriodicJobServiceImpl.lambda$1(PeriodicJobServiceImpl.java:90)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

[nblair]

Fixed the NPE in 0c173d8 - there's a requirement to call setBlobStore prior to start on the Metrics store.