Update travis to go1.7.3 (#109)

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 5307ea8..7f11d82 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 go:
   - 1.5.4
   - 1.6.3
-  - 1.7
+  - 1.7.3
   - tip
 matrix:
   allow_failures: