try runing before_install as a bat file...

.travis.yml

@@ -5,6 +5,8 @@ addons:
 
 import:
   - source: netfoundry/ziti-ci:ziti-ci.yml@may22-updates
+before_install:
+  - get-ziti-ci.bat
 
 language: go
 

get-ziti-ci.bat

@@ -0,0 +1,15 @@
+@echo off
+
+IF "x%ZITI_CI_VERSION%"=="x" GOTO DEFAULT
+
+echo Fetching ziti-ci@%ZITI_CI_VERSION%
+go get github.com/netfoundry/ziti-ci@%ZITI_CI_VERSION%
+GOTO END
+
+:DEFAULT
+echo Fetching default ziti-ci
+go get github.com/netfoundry/ziti-ci
+GOTO END
+
+:END
+echo go get of ziti-ci complete