TRAVIS_BUILD_DIR is the directory of kiwix-build.
So we must correctly set it in docker as the directory is not the
same.
We don't need the TRAVISCI_SSH_KEY now as we are looking in the right
directory.
kiwix-desktop needs the aria2 binary to download content.
kiwix-desktop will use aria2 throw lib-kiwix so it is somehow a kiwix-lib
dependency, but it is used only by kiwix-desktop.
And we know to install it only on platforms where we are supporting
kiwix-destkop.
On ubuntu, the libcurl package is compiled with libkrb (kerberos).
However, no libkrb5.a is provided and so, we cannot link statically.
As we are not using kerberos, we can (and must) compile ourselves libcurl.
