update ruby version matrix in travis config

Author: valich

.travis.yml

@@ -10,9 +10,10 @@ rvm:
   - 2.0.0
   - 2.1.10
   - 2.2.8
-  - 2.3.5
-  - 2.4.2
-  - 2.5.0
+  - 2.3.8
+  - 2.4.6
+  - 2.5.5
+  - 2.6.3
   - ruby-head
 
 matrix:
@@ -35,4 +36,4 @@ matrix:
 #      before_script: rvm install ruby-1.9.3 # not binary
 #    - os: osx
 #      rvm: 2.0.0
-#      before_script: rvm install ruby-2.0.0 # not binary
+#      before_script: rvm install ruby-2.0.0 # not binary