Reorder Rubies in Travis build CI

The first RVM entry is considered the default one.

This change has no impact on current build matrix, however if at some
point following would be added:

    matrix:
      include:
        - ...

Then it runs such additional job with default (first mentioned) Ruby
version unless that job definition specifies Ruby version explicitly.
Therefore, it's generally good when Rubies are listed from latest to
oldest.

.travis.yml

@@ -1,19 +1,19 @@
 language: ruby
 
 rvm:
-  - 1.9.3
-  - 2.0
-  - 2.1
-  - 2.2
-  - 2.3
-  - 2.4
-  - 2.5
   - 2.6
+  - 2.5
+  - 2.4
+  - 2.3
+  - 2.2
+  - 2.1
+  - 2.0
+  - 1.9.3
 
 matrix:
   allow_failures:
-    - rvm: 1.9.3
     - rvm: 2.0
+    - rvm: 1.9.3
 
 before_install:
   # this is a fix to get rng-tools to work in travis-ci