Upgrade Spring Framework to 5.1.15.RELEASE

Resolves gh-1636

Author: eleftherias

gradle/dependency-management.gradle

@@ -1,7 +1,7 @@
 dependencyManagement {
 	imports {
 		mavenBom 'io.projectreactor:reactor-bom:Californium-SR18'
-		mavenBom 'org.springframework:spring-framework-bom:5.1.14.RELEASE'
+		mavenBom 'org.springframework:spring-framework-bom:5.1.15.RELEASE'
 		mavenBom 'org.springframework.data:spring-data-releasetrain:Lovelace-SR16'
 		mavenBom 'org.springframework.security:spring-security-bom:5.1.8.RELEASE'
 		mavenBom 'org.testcontainers:testcontainers-bom:1.12.2'