Add drat folder to the valid repos

[zkamvar]

I had noticed that packages listed in the drat/ folder were not necessarily listed in the src/ folder, preventing their installation. If I added the drat/ folder to the valid repos, then these packages would install correctly.

[codecov]

Codecov Report

Merging #17 into master will decrease coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage     100%   99.57%   -0.43%     
==========================================
  Files           8        8              
  Lines         270      233      -37     
==========================================
- Hits          270      232      -38     
- Misses          0        1       +1

Impacted Files	Coverage Δ
R/r.R	97.91% <0%> (-2.09%)	⬇️
R/rstudio.R	100% <0%> (ø)	⬆️
R/main.R	100% <0%> (ø)	⬆️
R/build.R	100% <0%> (ø)	⬆️
R/config.R	100% <0%> (ø)	⬆️
R/nomad.R	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b52feab...38e3bd7. Read the comment docs.