Fix mongoskin version so install works

The dependency of mongoskin v0.4.4 was causing a wall of build errors:

``` bash
node-gyp clean
node-gyp configure build
  CXX(target) Release/obj.target/bson/ext/bson.o
In file included from ../ext/bson.cc:35:
../ext/bson.h:52:51: error: unknown type name 'Arguments'; did you mean 'v8::internal::Arguments'?
        static Handle<Value> BSONDeserializeStream(const Arguments &args);
                                                         ^~~~~~~~~
                                                         v8::internal::Arguments

many more lines here
```

It looks like there are some shenanigans going on with mongoskin, so I had to dial in on a version that worked and wasn't the latest version in npm (alpha on a minor release version with reported breaking changes / bugs, old versions seemingly removed).

This fix will make it so monk will install correctly.