Run tests vs Go 1.7, 1.8, and 1.9

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 go:
-    - 1.6
     - 1.7
     - 1.8
+    - 1.9
 install:
     - go get github.com/bwmarrin/discordgo
     - go get -v .