referencing system JavaScriptCore.framework

I changed plugin.xml to reference the system JavaScriptCore.framework (as opposed to the bundled JavaScriptCore.framework) as I was getting the following linker error building for iOS7 / iPad Air:

```
ld: warning: ignoring file /Users/alex/dev/imd/cordova/todo-lite/platforms/ios/TodoLite/Plugins/com.couchbase.lite.phonegap/JavaScriptCore.a, missing required architecture arm64 in file /Users/alex/dev/imd/cordova/todo-lite/platforms/ios/TodoLite/Plugins/com.couchbase.lite.phonegap/JavaScriptCore.a (3 slices)
Undefined symbols for architecture arm64:
  "_JSValueUnprotect", referenced from:
      -[CBLJSFunction dealloc] in libCBLJSViewCompiler.a(CBLJSFunction.o)
  "_JSValueMakeNumber", referenced from:
      _IDToValue in libCBLJSViewCompiler.a(CBLJSFunction.o)
  "_JSValueMakeString", referenced from:
...
```

The errors appear to be due to the arm64 architecture missing from the bundled version.