Issues in installation of dev version of ggmap library in R

I am using RStudio to with R 3.5.2 installed in my Windows 7 machine. I am trying to use ggmap to load google map based on certain parameters but this fails because the authentication key is not provided and a 403 error as below occurs

Error in download.file(url, destfile = tmp, quiet = !messaging, mode = "wb") : cannot open URL 'http://maps.googleapis.com/maps/api/staticmap?>center=29.763284,-95.363271&zoom=4&size=640x640&scale=2&maptype=hybrid&sensor=>false' In addition: Warning message: In download.file(url, destfile = tmp, quiet = !messaging, mode = "wb") : cannot open URL 'http://maps.googleapis.com/maps/api/staticmap?>center=29.763284,-95.363271&zoom=4&size=640x640&scale=2&maptype=hybrid&sensor=>false': HTTP status was '403 Forbidden'

A lot of posts suggests installing a dev version of ggmap after which you can use the maps API key by using

// save api key
register_google(key = "YOUR_API_KEY")

Thus I am trying to install the dev version using,

devtools::install_github("dkahle/ggmap")

but on trying to install the dev version of ggmap the error is as below

 devtools::install_github("dkahle/ggmap")
Error in curl::curl_fetch_memory(url, handle = h) : 
  schannel: next InitializeSecurityContext failed: SEC_E_UNTRUSTED_ROOT (0x80090325) - The certificate chain was issued by an authority that is not trusted.

any suggestions/help would be nice, Thanks