The package libcurl4-dev is a virtual package, and no package with that
name actually exists. Depend on libcurl4-openssl-dev as the primary
option in case no libcurl4-dev providing package has been installed, and
if some of them (libcurl4-gnutls-dev, libcurl4-nss-dev,
libcurl4-openssl-dev ) are installed, they also satisfy the dependency
via the virtual package.

The Travis-CI still runs on libcurl3-dev as Travis-CI also runs Ubuntu
Xenial which does not yet have libcurl4 available.

This complements commit 2e0a40bd.