Can't find libcurl or curl/curl.h (RuntimeError)

If you are on ubuntu:

sudo apt-get install libcurl3 libcurl3-gnutls libcurl4-openssl-dev

If you are ubuntu 18.04 LTS:

sudo apt-get install libcurl4 libcurl4-openssl-dev

The curb gem requires libcurl, something that is related to, but different from curl. It's the library that curl is built with. That missing file is part of the development version of cURL.