In ncbi_snp_query_api() convert from RJSONIO to rjsonlite

[jooolia]

In PR #92 the function ncbi_snp_query_api() was proposed which used RJSONIO to read into R the json file retrieved from the NCBI api. It is recommended that we use jsonlite instead for this conversion. This will require a few changes to the functions used in ncbi_snp_query_api() as jsonlite::fromJSON() returns dataframes compared to the lists in RJSONIO.