Upgrade to g@1.13

Author: tanx

.travis.yml

@@ -15,7 +15,7 @@ env:
   global:
     - LND_TAG=cd3c2b508ec45373602e3cdb9cd19ab7b224f94d
     - BTCD_TAG=16327141da8ce4b46b5bac57ba01352943465d9e
-    - GO_TAG=1.12.7
+    - GO_TAG=1.13
     - GOROOT=$HOME/go
     - GOPATH=$HOME/gocode
     - PATH=$GOPATH/bin:$GOROOT/bin:$PATH