Remove Go 1.5 from Travis

Fails to go get golint.

Also bump Go 1.7 to 1.7.3

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 0a7c136..707bdc3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,9 +3,8 @@
 language: go
 
 go:
-        - 1.5.4
         - 1.6.3
-        - 1.7
+        - 1.7.3
         - tip
 
 matrix: