MDL-66263 travis: Simple bump to php74

Also update a couple of places (mysql, node)
that were not runnig really on highest. Now they are.

.travis.yml

@@ -18,7 +18,7 @@ services:
 
 php:
     # We only run the highest and lowest supported versions to reduce the load on travis-ci.org.
-    - 7.3
+    - 7.4
     - 7.2
 
 addons:
@@ -48,11 +48,11 @@ jobs:
     fast_finish: true
 
     include:
-          # Run mysql only on 7.3 - it's just too slow
-        - php: 7.3
+          # Run mysql only on highest - it's just too slow
+        - php: 7.4
           env: DB=mysqli   TASK=PHPUNIT
-          # Run grunt/npm install on highest version ('node' is an alias for the latest node.js version.)
-        - php: 7.2
+          # Run grunt/npm install on highest version too ('node' is an alias for the latest node.js version.)
+        - php: 7.4
           env: DB=none     TASK=GRUNT   NVM_VERSION='lts/carbon'
 
 cache: