travis: remove testing for Laravel 5.8

It's officially not supported anymore since 2020-02-26

Author: mfn

.travis.yml

@@ -15,26 +15,18 @@ matrix:
   include:
     - php: '7.1'
       env: LARAVEL='5.5.*'
-    - php: '7.1'
-      env: LARAVEL='5.8.*'
     - php: '7.2'
       env: LARAVEL='5.5.*'
-    - php: '7.2'
-      env: LARAVEL='5.8.*'
     - php: '7.2'
       env: LARAVEL='^6.0'
     - php: '7.2'
       env: LARAVEL='dev-master'
     - php: '7.3'
       env: LARAVEL='5.5.*'
-    - php: '7.3'
-      env: LARAVEL='5.8.*'
     - php: '7.3'
       env: LARAVEL='^6.0' INSTALL_LARAVEL=1 INSTALL_LUMEN=1
     - php: '7.3'
       env: LARAVEL='dev-master'
-    - php: '7.4'
-      env: LARAVEL='5.8.*'
     - php: '7.4'
       env: LARAVEL='^6.0'
     - php: '7.4'