(MAINT) Update travis to test latest ruby

This commit updates travis to test latest ruby versions for vmpooler. Without this change we test out of date versions of ruby with vmpooler.

.travis.yml

@@ -6,28 +6,28 @@ services:
 
 matrix:
   include:
-    - rvm: 2.3.7
+    - rvm: 2.3.8
       env: "CHECK=rubocop"
 
-    - rvm: 2.3.7
+    - rvm: 2.3.8
       env: "CHECK=test"
 
-    - rvm: 2.4.4
+    - rvm: 2.4.5
       env: "CHECK=test"
 
-    - rvm: 2.5.1
+    - rvm: 2.5.3
       env: "CHECK=test"
 
     - rvm: jruby-9.1.17.0
       env: "CHECK=test"
 
-    - rvm: jruby-9.2.0.0
+    - rvm: jruby-9.2.5.0
       env: "CHECK=test"
 
   # Remove the allow_failures section once
   # Rubocop is required for Travis to pass a build
   allow_failures:
-    - rvm: 2.3.7
+    - rvm: 2.3.8
       env: "CHECK=rubocop"
 
 install: