Snapshot Tests use Framework 5.1.x

The 5.1.x code will only update patch versions of Spring Framework, so we
should lock down the Snapshot tests to use the latest 5.1.x changes

Fixes: gh-6880

Jenkinsfile

@@ -48,7 +48,7 @@ try {
 			node {
 				checkout scm
 				try {
-					sh "./gradlew clean test -PforceMavenRepositories=snapshot -PspringVersion='5.+' -PreactorVersion=Californium-BUILD-SNAPSHOT -PspringDataVersion=Lovelace-BUILD-SNAPSHOT --refresh-dependencies --no-daemon --stacktrace"
+					sh "./gradlew clean test -PforceMavenRepositories=snapshot -PspringVersion='5.1.+' -PreactorVersion=Californium-BUILD-SNAPSHOT -PspringDataVersion=Lovelace-BUILD-SNAPSHOT --refresh-dependencies --no-daemon --stacktrace"
 				} catch(Exception e) {
 					currentBuild.result = 'FAILED: snapshots'
 					throw e