Use go12 in CI

.travis.yml

@@ -11,7 +11,13 @@ matrix:
     - python: 3.7
       dist: xenial
       env: TOXENV=py37-interop
+      sudo: true
       before_install:
+        - wget https://dl.google.com/go/go1.12.6.linux-amd64.tar.gz
+        - sudo tar -C /usr/local -xzf $GOPACKAGE
+        - export GOPATH=$HOME/go
+        - export GOROOT=/usr/local/go
+        - export PATH=$GOROOT/bin:$GOPATH/bin:$PATH
         - ./install_interop_go_pkgs.sh
 
 install:

install_interop_go_pkgs.sh

@@ -1,13 +1,5 @@
 #!/bin/bash
 
-which go
-if [ $? != 0 ]; then
-    wget https://dl.google.com/go/go1.12.6.linux-amd64.tar.gz
-    sudo tar -C /usr/local -xzf $GOPACKAGE
-    export GOPATH=$HOME/go
-    export GOROOT=/usr/local/go
-fi
-
 go version
 cd tests/interop/go_pkgs/
 go install ./...