refactor(ci): update libvips versions

Author: h2non

.travis.yml

@@ -7,19 +7,18 @@ services:
   - docker
 
 env:
-  - LIBVIPS=7.42.3
+  # - LIBVIPS=7.42.3
   # - LIBVIPS=8.4.6
   # - LIBVIPS=8.5.8
   # - LIBVIPS=8.6.2
   # - LIBVIPS=8.7.4
-  # - LIBVIPS=8.8.4
+  - LIBVIPS=8.8.4
   - LIBVIPS=8.9.2
   - LIBVIPS=master
 
 matrix:
   allow_failures:
     - env: LIBVIPS=7.42.3
-    - env: LIBVIPS=8.4.6
     - env: LIBVIPS=master
 
 cache: