Update travis config to use latest go versions (#540)

diff --git a/.travis.yml b/.travis.yml
index d9885c6..055480b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,16 +1,16 @@
 language: go
 
 go:
-    - "1.4"
-    - "1.5"
-    - "1.6"
-    - "1.7"
-    - "1.8"
-    - "1.9"
-    - "1.10"
-    - "1.11"
-    - "1.12"
-    - "1.13"
+    - "1.4.x"
+    - "1.5.x"
+    - "1.6.x"
+    - "1.7.x"
+    - "1.8.x"
+    - "1.9.x"
+    - "1.10.x"
+    - "1.11.x"
+    - "1.12.x"
+    - "1.13.x"
     - "tip"
 
 go_import_path: gopkg.in/yaml.v2