Update Travis to run Go 1.7 and 1.8 versions (#22)

diff --git a/.travis.yml b/.travis.yml
index f1ce95a..fc32b6f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,8 @@
 go_import_path: go.uber.org/atomic
 
 go:
-  - 1.5
-  - 1.6
+  - 1.7
+  - 1.8
   - tip
 
 cache: