Add Explicit rJava Dependency for Package Developers

[BenjaminWolfe]

I've always been able to use your package in R scripts by following the README.

However, when I wrote a simple wrapper package for my organization, which made use of your package (and others) and added some company defaults, helpful messaging, etc., I got this error:

Error in .jinit() : could not find function ".jinit"

In a simple script, this is solved by including the right library calls before loading your package. But that's not something you can do with a package, since it has to be more disciplined around the use of namespaces.

I've already submitted a pull request, PR 13, which solves this issue, has been tested for a year and a half, and can be merged right now.

(When I say it's been tested, I mean I've been using my own fork and have observed no side effects.)

I'll quickly list the changes here, for reference:

• Add rJava to the dependencies list in the DESCRIPTION file.
• Fully qualify rJava::.jinit() and rJava::.jcall() with double-colon notation in lines 189 and 190, respectively.
• Optionally, correct the spelling of "dependencies" in the [README](https://github.com/snowflakedb/dplyr-snowflakedb/blob/master/README.md#r-environment-setup).