cli: diff sections don't maintain the order given by the server

[BrunoRosendo]

This happens because, in contrary to Python's dictionaries, GO's maps are not ordered.

Here's an example:

$ reana-client diff events.1 test | grep "^==>"
==> Differences in workflow inputs
==> Differences in workflow outputs
==> Differences in workflow version
==> Differences in workflow workspace
==> Differences in workflow specification
==> Differences in workflow workspace

$ $ ./reana-client-go diff events.1 test | grep "==>"
==> Differences in workflow specification
==> Differences in workflow inputs
==> Differences in workflow outputs
==> Differences in workflow version
==> Differences in workflow workspace
==> Differences in workflow workspace

This problem was discussed in this comment: #82 (comment)
Some possible solutions include using json.Decoder to decode the JSON object manually, or use a library like orderedmap to ensure the order is maintained.