boot2docker / fix broken get last release version function if curl UA is set to IE

a `curl -A "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)" 'https://api.github.com/repos/steeve/boot2docker/releases' `
this will return a JSON with windows line feeds which did not get correctly parsed

add more error handling

fixes this #106