[bootstrap] update brew install command (#574)
[bootstrap] update brew install command (#574)

Warning: The Ruby Homebrew installer is now deprecated and has been rewritten in
Bash. Please migrate to the following command:
  /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
jwhui/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,bukepo/borderrouter,openthread/borderrouter,jwhui/borderrouter,bukepo/borderrouter,bukepo/borderrouter,jwhui/borderrouter,jwhui/borderrouter,openthread/ot-br-posix,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,jwhui/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix