Remove go 1.3 and 1.4 from travis; add 1.6

Due to the age of 1.3 and 1.4, removing them from travis.  They
currently are incompatible with golint as well, meaning that we,
otherwise, need to remove golint in testing.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index c7d8e05..780c7ff 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,9 +3,8 @@
 language: go
 
 go:
-        - 1.3
-        - 1.4
         - 1.5
+        - 1.6
         - tip
 
 install: