v0.6.7, readline(::HTTP.Stream) and 0.7.0-DEV.4616 compat

Require MbedTLS.jl v0.5.7

Add Base.readuntil(::HTTP.Stream, ::UInt8)

Fix IOExtras.unread!(::HTTP.Stream)

Now Base.readline(::HTTP.Stream) works

compat for 0.7.0-DEV.4616