travis: update travis build to go1.8.1

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go:
   - 1.7.5
-  - 1.8
+  - 1.8.1
 sudo: false
 install:
   - go get -v github.com/roasbeef/btcd # Required for the rpc test package.