Sorry; I missed that in your original post.
I was able to download lmtest just now:
> install.packages("lmtest")
Installing package into '/Users/kevin/Library/R/3.4/library'
(as 'lib' is unspecified)
trying URL 'https://cran.rstudio.org/bin/macosx/el-capitan/contrib/3.4/lmtest_0.9-36.tgz'
Content type 'application/x-gzip' length 277347 bytes (270 KB)
==================================================
downloaded 270 KB
And the CRAN webpage suggests that package was just updated:
https://cran.r-project.org/web/packages/lmtest/index.html
So you might need to restart R and just try installation again once more.