rm go1.5.4 from travis config since the linter doesn't build in 1.5 (#175)

diff --git a/.travis.yml b/.travis.yml
index e6def9a..3a5c933 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,6 @@
 language: go
 
 go:
-  - 1.5.4
   - 1.6.3
   - tip