Make hamcrest dependency optional in spring-test

Users of Spring MVC Test will need to list a hamcrest dependency --
either hamcrest-library or hamcrest-all.

Issue: SPR-9940

build.gradle

@@ -557,7 +557,8 @@ project('spring-test-mvc') {
         compile project(":spring-webmvc")
         compile project(":spring-test").sourceSets.main.output
         compile("org.apache.tomcat:tomcat-servlet-api:7.0.8", provided)
-        compile "org.hamcrest:hamcrest-library:1.3"
+        compile("org.hamcrest:hamcrest-core:1.3", optional)
+        compile("org.hamcrest:hamcrest-library:1.3", optional)
         compile("com.jayway.jsonpath:json-path:0.8.1", optional)
         compile("xmlunit:xmlunit:1.2", optional)
         testCompile("org.slf4j:jcl-over-slf4j:1.6.1")