Update CI

.travis.yml

@@ -8,6 +8,7 @@ language: cpp
 
 matrix:
   include:
+    - if: tag IS present
       - os: linux
         dist: xenial
         sudo: required

appveyor.yml

@@ -12,6 +12,8 @@ environment:
       qtdir: C:\Qt\5.13.2\msvc2017
       sysdirname: system32
       
+  skip_non_tags: true
+
 cache:
   CGAL-5.1-Setup.exe -> appveyor.yml