don't force dependencies = TRUE in install_github()

[krlmlr]

No description provided.

[hadley]

That wasn't a regression it was a deliberate change. The problem is that there's two uses of install github:

1. For user. Needs vignettes to be built so needs suggested packages.
2. For developer or as prereq. Doesn't need vignettes. But probably want suggested packages so that you can run all tests.

I'm not sure how to better differentiate these use cases.

[krlmlr]

I'm fine with saying dep=NA if I don't need the suggested packages. The problem is that now the dependencies parameter is ignored -- install_github_single is called with dependencies = TRUE, and this PR fixes it.

[hadley]

Oh got it!