refactor: enable automatic iOS builds (#209)

* refactor: enable automatic iOS builds

Part of https://github.com/ooni/probe/issues/1335

* fix: go mod tidy

build-ios.bash

@@ -0,0 +1,11 @@
+#!/bin/bash
+set -e
+topdir=$(cd $(dirname $0) && pwd -P)
+set -x
+export PATH=$(go env GOPATH)/bin:$PATH
+export GO111MODULE=off
+go get -u golang.org/x/mobile/cmd/gomobile
+gomobile init
+export GO111MODULE=on
+output=MOBILE/ios/oonimkall.framework
+gomobile bind -target=ios -o $output -ldflags="-s -w" ./pkg/oonimkall