Removes branch "prosecutor" for earlywarning pkg.

Using the `install_github("earlywarning")` command with `branch="prosecutor"` parameter yields an error for me, whereas without the branch it does not. Perhaps there needs to be a branch parameter, but as given it doesn't work for me.

```
> install_github("earlywarning", "cboettig", branch="prosecutor")
Installing github repo earlywarning/prosecutor from cboettig
Downloading earlywarning.zip from https://github.com/cboettig/earlywarning/archive/prosecutor.zip
Installing package from /var/folders/fq/dfzxfytx7fz_1cvd843nklm00000gq/T//RtmppRk27B/earlywarning.zip
arguments 'minimized' and 'invisible' are for Windows only
Installing earlywarning
'/Library/Frameworks/R.framework/Resources/bin/R' --vanilla CMD INSTALL  \
  '/private/var/folders/fq/dfzxfytx7fz_1cvd843nklm00000gq/T/RtmppRk27B/devtools686046451dc5/earlywarning-prosecutor'  \
  --library='/Library/Frameworks/R.framework/Versions/3.0/Resources/library'  \
  --install-tests 

* installing *source* package 'earlywarning' ...
** R
Error: Command failed (1)
In addition: Warning message:
In FUN("earlywarning"[[1L]], ...) :
  'branch' is deprecated. In the future, please use 'ref' instead.
Error in .install_package_code_files(".", instdir) : 
files in '/private/var/folders/fq/dfzxfytx7fz_1cvd843nklm00000gq/T/RtmppRk27B/devtools686046451dc5/earlywarning-prosecutor/R' missing from 'Collate' field:
  bayeswarning.R
ERROR: unable to collate and parse R files for package 'earlywarning'
* removing '/Library/Frameworks/R.framework/Versions/3.0/Resources/library/earlywarning'
```