CI: disable the installation of dmd
Currently all the builds fail:
$ mkdir -p ~/dlang && wget https://dlang.org/install.sh -O ~/dlang/install.sh
--2021-01-10 06:05:39-- https://dlang.org/install.sh
Resolving dlang.org (dlang.org)... 162.217.114.56, 2607:fc50:1:ff02::5:0
Connecting to dlang.org (dlang.org)|162.217.114.56|:443... connected.
ERROR: cannot verify dlang.org's certificate, issued by ‘CN=R3,O=Let's Encrypt,C=US’:
Unable to locally verify the issuer's authority.
To connect to dlang.org insecurely, use `--no-check-certificate'.
* .travis.yml: Disable dmd.