Installing RSelenium from GitHub

I want to use RSelenium for R, but it has been removed from CRAN. I decided to follow the guides using a download/installation from GitHub but I get an error message that I cant explain.

devtools::install_github("ropensci/RSelenium")

R Starts downloading

Downloading GitHub repo ropensci/RSelenium@master
These packages have more recent versions available.
Which would you like to update?

1: All                             
2: CRAN packages only              
3: None                            
4: XML (3.98-1.20 -> 3.99-0) [CRAN]

Enter one or more numbers, or an empty line to skip updates:

Im entering 1, because I want everything updated.

XML        (3.98-1.20 -> 3.99-0) [CRAN]
wdman      (NA        -> 0.2.4 ) [CRAN]
binman     (NA        -> 0.1.1 ) [CRAN]
subprocess (NA        -> 0.8.3 ) [CRAN]
semver     (NA        -> 0.2.0 ) [CRAN]
Installing 5 packages: XML, wdman, binman, subprocess, semver

Error: Failed to install 'RSelenium' from GitHub:
  (converted from warning) unable to access index for repository http://www.omegahat.net/R/bin/macosx/el-capitan/contrib/3.6:
  cannot open URL 'http://www.omegahat.net/R/bin/macosx/el-capitan/contrib/3.6/PACKAGES'

How can I fix that issue? (My version of R and RStudio are updated, thats what usually makes packages not work for me. The problem fixer no1 "Restart R Studio" didn't help either ;) ).