Import `jsonlite::fromJSON()`

[joshuaulrich]

Using jsonlite::fromJSON() while jsonlite is only in Suggests causes a failure in the tseries package examples because tseries does not suggest jsonlite.

Running _R_CHECK_SUGGESTS_ONLY_=true R CMD check tseries --as-cran causes getSymbols.yahoo() to fail.

We need to import jsonlite::fromJSON() to fix.

Thanks to Kurt Hornik for the report and fix!