Bioconductor dependencies from package description

This will make the support for Bioconductor package development more similar to CRAN. I have not yet included installing the devel version of R (nightly snapshot), which is what some developers use for CRAN and sometimes Bioconductor; we can do that in the future.
This command fails because it is missing `devtools`. The fix is to add:

```
EnsureDevtools
```
just before line 345.

An example of this failing can be seen at https://travis-ci.org/lcolladotor/derfinder/builds/35174073