Fix travis.yaml reference to Go 1.10.

.travis.yml

@@ -1,15 +1,15 @@
 language: go
 
 go:
-    - 1.4
-    - 1.5
-    - 1.6
-    - 1.7
-    - 1.8
-    - 1.9
-    - 1.10
-    - 1.11
-    - 1.12
+    - "1.4"
+    - "1.5"
+    - "1.6"
+    - "1.7"
+    - "1.8"
+    - "1.9"
+    - "1.10"
+    - "1.11"
+    - "1.12"
     - tip
 
 go_import_path: gopkg.in/yaml.v2