Use r6

[crew102]

• Closes #90
• I've stacked this PR on top of #137 so that tests run properly (hence the commits https://github.com/daattali/ggExtra/pull/138/commits/3de4a5ab1c21406c4ae5f0ac23633034aaa2aa5d - https://github.com/daattali/ggExtra/pull/138/commits/b0de213bc71b7c79d6b300a771e5992cc277740e appearing in this PR). It shouldn't matter that those commits are in this PR, as long as you squash before merging.
• I would wait until after #137 is merged and Travis finishes the build before you attempt to merge this into master - weird things seems to happen when builds overlap for ggExtra.

[daattali]

LGTM (at a later point I might care to revisit the logic inside the public build() method to try to make some more use of it being in an object oriented world)