added `dependencies = TRUE`

Seems that the vignettes require a bunch of packages I'd never heard of and the install failed several times in a row because I didn't have them, one after the other... It will simplify the package install if those other packages are installed all at once rather than going through the cycle of install bf -> install fails due to missing package -> install missing package -> attempt to install bf again -> install fails due to missing package etc.
Merge pull request #11 from benmarwick/patch-1

added `dependencies = TRUE`