Pretty print JSON data

See travis-ci/travis-ci#1555.

JSON data are pretty-formatted for humans.
Accept `pretty` parameter for pretty formatting

If the parameter is equal to `true` (in any case--`TRUE`, `True`) or
a positive integer, return pretty formatted JSON data.

No tests are necessary.
Account for absence of `pretty` parameter
Add specs for pretty print JSON

They only check that the response includes `\n`, which should not happen
otherwise.
Merge pull request #97 from travis-ci/core-gh1555-pretty-print-json

Pretty print JSON data