CI: show the version of the tools we use

[Akim Demaille]

On master.

 commit ec99c418a7c19bba2b87456580906d2fb1a7bb3c
 Author: Akim Demaille <address@hidden>
 Date:   Fri Sep 7 06:17:17 2018 +0200

    CI: show the version of the tools we use
    
    We have failures on Flex output, which are probably related to an old
    release.  Let's check.
    
        In file included from src/scan-code-c.c:3:
        src/scan-code.c:2198:21: error: empty paragraph passed to '@param' 
command
              [-Werror,-Wdocumentation]
         * @param line_number
           ~~~~~~~~~~~~~~~~~^
    
    * .travis.yml: here.

diff --git a/.travis.yml b/.travis.yml
index b27569cd..6eeb3b0e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -118,6 +118,16 @@ matrix:
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -qq autoconf automake autopoint flex gettext graphviz 
help2man m4 texinfo valgrind
+  - autoconf --version
+  - automake --version
+  - autopoint --version
+  - flex --version
+  - gettext --version
+  - dot -V
+  - help2man --version
+  - m4 --version
+  - makeinfo --version
+  - valgrind --version
   - eval "${MATRIX_EVAL}"
 
 script: