Fix test deprecation warning on 0.6

JSON v0.8.0 [0e36473]

JSON v0.7.1 [1ec27df]

JSON v0.7.0 [21dcd64]

JSON v0.6.1 [38f4d37]

JSON v0.5.4 [6df70b0]

JSON v0.6.0 [1082e64]

JSON v0.5.2 [1082e64]

Merge pull request JuliaIO#149 from TotalVerb/pass-the-tests-without-…

…warnings

Fix @os_only deprecation warnings

JSON v0.5.1 [6df70b0]