plugins/boot2docker: use `$OSTYPE` instead of `$(uname)`

plugins/available/boot2docker.plugin.bash

@@ -3,7 +3,7 @@ about-plugin 'Helpers to get Docker setup correctly for boot2docker'
 
 # Note, this might need to be different if you have an older version
 # of boot2docker, or its configured for a different IP
-if [[ `uname -s` == "Darwin" ]]; then
+if [[ "$OSTYPE" == 'darwin'* ]]; then
   export DOCKER_HOST="tcp://192.168.59.103:2376"
   export DOCKER_CERT_PATH="~/.boot2docker/certs/boot2docker-vm"
   export DOCKER_TLS_VERIFY=1