optimize execFunctionDeps loading in worker.R

using `rawConnection` to keep the `load` in memory;
see http://stackoverflow.com/a/23597894/4134338
Indeed there could be issues (#55). I'm more referring to just switching the order we write out packages and function dependencies in [here](https://github.com/amplab-extras/SparkR-pkg/blob/master/pkg/src/src/main/scala/edu/berkeley/cs/amplab/sparkr/RRDD.scala#L141). If it's not too much troubles could you include this optimization as well?