Jenkinsfile uses Spring 5.1.0.BUILD-SNAPSHOT

Previously 5.+ was used. This caused problems because Spring Framework
does not use semantic versioning for milestone and rc snapshots. For
example, Spring uses 5.1.0.BUILD-SNAPSHOT, 5.1.0.M1, and then goes back
to 5.1.0.BUILD-SNAPSHOT.

Fixes: gh-5721

Jenkinsfile

@@ -44,7 +44,7 @@ try {
 			node {
 				checkout scm
 				try {
-					sh "./gradlew clean test -PspringVersion='5.+' -PreactorVersion=Californium-BUILD-SNAPSHOT -PspringDataVersion=Lovelace-BUILD-SNAPSHOT --refresh-dependencies --no-daemon --stacktrace"
+					sh "./gradlew clean test -PspringVersion='5.1.0.BUILD-SNAPSHOT' -PreactorVersion=Californium-BUILD-SNAPSHOT -PspringDataVersion=Lovelace-BUILD-SNAPSHOT --refresh-dependencies --no-daemon --stacktrace"
 				} catch(Exception e) {
 					currentBuild.result = 'FAILED: snapshots'
 					throw e