optimize execFunctionDeps loading in worker.R

using `rawConnection` to keep the `load` in memory;
see http://stackoverflow.com/a/23597894/4134338
@concretevitamin 
it's not necessary to `close` the rawConnection explicitly, because it would happen automatically as the documentation of `connection` says:

> close closes and destroys a connection. This will happen automatically in due course (with a warning) if there is no longer an R object referring to the connection.

I omitted `close` here to leave the one-line code compact.