.travis.yml: attempt to fix broken CI

[...]
$ sudo apt-get install cmake valgrind
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 valgrind : Depends: libc6-dbg but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
The command "sudo apt-get install cmake valgrind" failed and exited with 100 during .

.travis.yml

@@ -1,6 +1,12 @@
 language: c
 compiler: gcc
 
+install:
+    - sudo apt-get install cmake valgrind
+
+script:
+    - make distcheck
+
 matrix:
     include:
         - name: "Ubuntu LTS 2014 (trusty)"
@@ -14,12 +20,9 @@ matrix:
 
         - name: "Ubuntu LTS 2020 (focal)"
           dist: focal
-
-install:
-    - sudo apt-get install cmake valgrind
-
-script:
-    - make distcheck
+          # work around broken Ubuntu repos (libc6-dbg does not match libc6 version)
+          install:
+            - sudo apt-get install cmake
 
 env:
     global: