Travis: fix that major.minor version doesn't track latest major.minor.tiny

So `2.4` would match `2.4.1` (due to an rvm alias) rather than matching
the latest `2.4.3` release.

.travis.yml

@@ -60,21 +60,21 @@ env:
     - "GEM=ac:integration"
 
 rvm:
-  - 2.4
-  - 2.5
+  - 2.4.3
+  - 2.5.0
   - ruby-head
 
 matrix:
   include:
-    - rvm: 2.5
+    - rvm: 2.5.0
       env: "GEM=av:ujs"
-    - rvm: 2.4
+    - rvm: 2.4.3
       env: "GEM=aj:integration"
       services:
         - memcached
         - redis-server
         - rabbitmq
-    - rvm: 2.5
+    - rvm: 2.5.0
       env: "GEM=aj:integration"
       services:
         - memcached
@@ -86,15 +86,15 @@ matrix:
         - memcached
         - redis-server
         - rabbitmq
-    - rvm: 2.5
+    - rvm: 2.5.0
       env:
         - "GEM=ar:mysql2 MYSQL=mariadb"
       addons:
         mariadb: 10.2
-    - rvm: 2.5
+    - rvm: 2.5.0
       env:
         - "GEM=ar:sqlite3_mem"
-    - rvm: 2.5
+    - rvm: 2.5.0
       env:
         - "GEM=ar:postgresql POSTGRES=9.2"
       addons: