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
Remove unnecessary spring-instrument dependencies

Remove unnecessary spring-core dependency from spring-instrument.

Issue: SPR-10732