fix(response): handle empty string commits

lostisland/faraday@f41ffaabb72d3700338296c79a2084880e6a9843 changed the
net-http adapter to return empty strings for empty bodies instead of
nil, which then causes the JSON parser to error when attempting to parse
it, since an empty string is not valid JSON.
Merge pull request #12 from travis-ci/hh-handle-empty-string-bodies

fix(response): handle empty string commits