Greener Travis? (#2589)

With PHP 7.4 just around the corner, I suggest to test only the minimum
and maximum PHP versions we support in Travis, to save resources
(greener).
For the translation check, with can test with max version -1 to run on
yet another PHP version.
It makes also the Travis tests faster to complete.

.travis.yml

@@ -1,9 +1,6 @@
 language: php
 php:
   - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
   - 7.3
 
 install:
@@ -51,4 +48,4 @@ matrix:
         - git ls-files --exclude='*Dockerfile*' --ignored | xargs --max-lines=1 "$HADOLINT"
   allow_failures:
     - env: CHECK_TRANSLATION=yes VALIDATE_STANDARD=no
-    - dist: precise
+