Update go versions for testing

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index ddb9a8c..7144fd2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: go
 go:
     - 1.4.3
-    - 1.5.3
-    - 1.6.3
-    - 1.7.3
+    - 1.5.4
+    - 1.6.4
+    - 1.7.4
+    - 1.8rc1