Bump Go versions, use latest patch releases (#110)

.travis.yml

@@ -1,10 +1,11 @@
 language: go
 go_import_path: github.com/pkg/errors
 go:
-  - 1.4.3
-  - 1.5.4
-  - 1.6.3
-  - 1.7.3
+  - 1.4.x
+  - 1.5.x
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
   - tip
 
 script: