Add Java 7 instrumentation manifest attributes

Update META-INF/MANIFEST.MF for spring-instrument to include
necessary attributes for running under Java 7:

    Can-Redefine-Classes : true
    Can-Retransform-Classes: true
    Can-Set-Native-Method-Prefix : false

(see http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/
package-summary.html)

Prior to this commit `InstrumentationSavingAgent.getInstrumentation().
addTransformer(t, true);` would fail under Java 7.

Issue: SPR-10731
(Cherry picked from 50333ca)

Author: philwebb

build.gradle

@@ -297,6 +297,9 @@ project("spring-instrument") {
 	jar {
 		manifest.attributes["Premain-Class"] =
 			"org.springframework.instrument.InstrumentationSavingAgent"
+		manifest.attributes["Can-Redefine-Classes"] = "true"
+		manifest.attributes["Can-Retransform-Classes"] = "true"
+		manifest.attributes["Can-Set-Native-Method-Prefix"] = "false"
 	}
 }