travis: update go versions

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 go:
-    - 1.7
-    - 1.8
-    - 1.9
+    - 1.7.x
+    - 1.8.x
+    - 1.9.x
 install:
     - go get github.com/bwmarrin/discordgo
     - go get -v .