Fixing incompatibilties with Pebble

[kmadac]

Pebble is official lightweight test server from Letsencrypt.

One difference between boulder and pebble is that http header names start with lower-case instead of upper-case. As this behavior is in compliance with RFC acme.sh should be able to work in both cases.

Another fix is that certificate field is not part of finalize-order response, but we need to get certificate url from request to order URL taken from Location header.

[kmadac]

Travis CI job has been stucked on brew update && brew install socat for 10 minutes and then it was killed by travis. Can anybody run the build again?

[Neilpang]

grep -i is ok.

can you update this PR ?