在安装过程中，我尝试下载R中的GGMAP软件包，同时还会下载ggplot 2.1.0。我收到一个错误，但是ggmap所需的ggplot版本是2.2.0。如何强制ggplot为最新版本？
> library(devtools)
> install_github("dkahle/ggmap")
Downloading GitHub repo dkahle/ggmap@master
from URL https://api.github.com/repos/dkahle/ggmap/zipball/master
Installing ggmap
"C:/PROGRA~1/MIE74D~1/RCLIEN~1/R_SERVER/bin/x64/R" --no-site-file --no-environ --no-save --no-restore --quiet CMD INSTALL \
"C:/Users/dixi/AppData/Local/Temp/RtmpuYriTy/devtools2604499b1ebd/dkahle-ggmap-065fe6a" \
--library="C:/Users/dixi/Documents/R/win-library/3.3" --install-tests
* installing *source* package 'ggmap' ...
** R
** data
*** moving datasets to lazyload DB
** inst
** tests
** preparing package for lazy loading
Error : package 'ggplot2' 2.1.0 was found, but >= 2.2.0 is required by 'ggmap'
ERROR: lazy loading failed for package 'ggmap'
* removing 'C:/Users/dixi/Documents/R/win-library/3.3/ggmap'
* restoring previous 'C:/Users/dixi/Documents/R/win-library/3.3/ggmap'
Error: Command failed (1)