Add some prior go versions

We've got code that works around some older versions, so let's verify them.
diff --git a/.travis.yml b/.travis.yml
index ec23bc5..7ea0357 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,9 @@
 sudo: false
 language: go
 go:
+  - 1.3.3
+  - 1.4.3
+  - 1.5.4
   - 1.6.2
   - tip
 matrix: