CI: disable the installation of dmd

[Akim Demaille]

I don't know what happens, and I have still not spent time on moving from dmd 
to the other compilers.

commit b1f03fe577063194c454f26910223fdb07ec920a
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Sun Jan 10 07:36:54 2021 +0100

    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.

diff --git a/.travis.yml b/.travis.yml
index c2e61f35..dfdf6ddd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -505,12 +505,12 @@ script:
   # - env
   - sudo apt-get install -qq doxygen flex m4
   # Install and activate dmd.
-  - mkdir -p ~/dlang && wget https://dlang.org/install.sh -O ~/dlang/install.sh
-  - source $(source ~/dlang/install.sh dmd -a) || true
+  #- mkdir -p ~/dlang && wget https://dlang.org/install.sh -O 
~/dlang/install.sh
+  #- source $(source ~/dlang/install.sh dmd -a) || true
 
   - $CC --version
   - $CXX --version
-  - dmd --version || true
+  #- dmd --version || true
   - doxygen --version
   - flex --version
   - ld --version