Use cacerts on verify peer

[josevalim]

Otherwise SSL fails, for example:

iex(1)> :ssl.connect ~c"google.com", 443, [verify: :verify_peer]
{:error, {:options, :incompatible, [verify: :verify_peer, cacerts: :undefined]}}

[josevalim]

💚 💙 💜 💛 ❤️

[josevalim]

I reverted this. The behaviour only happens for ssl: true in new Erlang, so should only change the default in that branch. However, changing it in there will impact other Erlang/OTP versions.