[smuxi: 35/37] Travis-CI: drop legacy Mono versions

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit b26af16b991e22d977cc67373d3c4a925a1b3197
Author: Mirco Bauer <meebey meebey net>
Date:   Sun Jan 3 23:42:39 2021 +0800

    Travis-CI: drop legacy Mono versions
    
    Removed all versions except current Debian stable+oldstable and Ubuntu
    LTS+previous.
    
    No entry for Debian 10 (buster) which ships Mono 5.18.0 as that version doesn't
    install, but errors with:
    
        12.71s0.37sE: Unable to locate package referenceassemblies-pcl
        The command "sudo apt-get install -qq mono-complete mono-vbnc fsharp nuget referenceassemblies-pcl" 
failed and exited with 100 during .
    
    See https://travis-ci.org/github/meebey/smuxi/jobs/752624693

 .travis.yml | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)
---
diff --git a/.travis.yml b/.travis.yml
index 6ac47122..6e9dfbbb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,12 +6,8 @@ before_install:
 
 mono:
     - latest
-    - 4.6.2
-    - 4.2.3
-    - 4.2.1
-    - 4.0.0
-    - 3.12.0
-    - 2.10.8
+    - 6.8.0   # Ubuntu 20.04 LTS (focal)  / Debian bullseye
+    - 4.6.2   # Ubuntu 18.04 LTS (bionic) / Debian 9 (strech)
 
 addons:
     apt: