Make spring-core optional

It isn't really optional, but no-one uses Spring Retry without it
and some older build systems (Gradle old versions) don't like finding
multiple versions of thw same library.

pipeline.yml

@@ -88,7 +88,7 @@ jobs:
         - |
           rm -rf ~/.m2
           ln -s $(pwd)/.m2 ~/.m2
-          ./mvnw install -s .mvn/settings.xml -Dspring.framework.version=5.0.0.RC2
+          ./mvnw install -s .mvn/settings.xml -Dspring.framework.version=5.3.2
     params:
       TERM: dumb
       passphrase: {{passphrase}}

pom.xml

@@ -262,6 +262,8 @@
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-core</artifactId>
+			<!-- Not really optional, but since it works with 4 and 5 this is safe -->
+			<optional>true</optional>
 		</dependency>
 		<dependency>
 			<groupId>org.springframework</groupId>