Run kustomize tests on OSX

.travis.yml

@@ -1,3 +1,15 @@
+os:
+# TODO: Enable this when we can get the tests to work
+#  - windows
+  - linux
+  - osx
+
+addons:
+  apt:
+    packages: tree
+  homebrew:
+    packages: tree
+
 language: go
 
 go:
@@ -17,7 +29,6 @@ env:
 
 before_install:
   - source ./bin/consider-early-travis-exit.sh
-  - sudo apt-get install tree
   - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin ${GOLANGCI_RELEASE}
   - go get -u github.com/monopole/mdrip