[smuxi/stable: 13/15] Travis-CI: remove Mono 5.18 from versions to fix build

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

commit 5459e583285efa75deb766d3d2bf37003dca105b
Author: Mirco Bauer <meebey meebey net>
Date:   Mon Jun 15 22:42:36 2020 +0800

    Travis-CI: remove Mono 5.18 from versions to fix build
    
    Debian 10 (buster) is shipped with Mono 5.18 but unfortunately Travis' C#
    support doesn't have that version. The build errors with:
    
        Installing Mono
        E: 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 .

 .travis.yml | 1 -
 1 file changed, 1 deletion(-)
---
diff --git a/.travis.yml b/.travis.yml
index a2903514..e1e20b89 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,6 @@ before_install:
 mono:
     - latest
     - 6.8.0
-    - 5.18.0
     - 4.6.2
     - 4.2.3
     - 4.2.1