Upgrade to go v1.12.1

Author: tanx

.travis.yml

@@ -15,7 +15,7 @@ env:
   global:
     - LND_TAG=10910318a2cdb7f7d0954cfc0de2eea96abf00cf
     - BTCD_TAG=aa6e0f35703c1438cc45860ddc0c3f6e6c633c93
-    - GO_TAG=1.11.5
+    - GO_TAG=1.12.1
     - GOROOT=$HOME/go
     - GOPATH=$HOME/gocode
     - PATH=$GOPATH/bin:$GOROOT/bin:$PATH