Correct excluded Ruby versions

The versions we were running against were bumped, so we needed to update
which versions were being excluded as they were stale.

Rails master (soon to be 6.0.0) doesn't support versions before Ruby
2.5.

.travis.yml

@@ -33,7 +33,7 @@ matrix:
   exclude:
     - rvm: 2.2
       env: RAILS='master'
-    - rvm: 2.3.5
+    - rvm: 2.3.8
       env: RAILS='master'
-    - rvm: 2.4.3
+    - rvm: 2.4.5
       env: RAILS='master'