Add missing var in front of j$ to avoid polluting global namespace

Users should only have 'jasmine' in global namespace, j$ should be
fully internal unless developing for jasmine

As brought up by Morten Maxild

lib/jasmine-core/jasmine.js

@@ -30,7 +30,7 @@ function getJasmineRequireObj() {
 }
 
 getJasmineRequireObj().core = function(jRequire) {
-  j$ = {};
+  var j$ = {};
 
   jRequire.base(j$);
   j$.util = jRequire.util();

src/core/requireCore.js

@@ -8,7 +8,7 @@ function getJasmineRequireObj() {
 }
 
 getJasmineRequireObj().core = function(jRequire) {
-  j$ = {};
+  var j$ = {};
 
   jRequire.base(j$);
   j$.util = jRequire.util();