Update tested versions.

.travis.yml

@@ -1,8 +1,9 @@
 language: go
 go:
-    - 1.9.x
     - 1.10.x
     - 1.11.x
+    - 1.12.x
+    - 1.13.x
 install:
     - go get github.com/bwmarrin/discordgo
     - go get -v .