replace jsonlite base64_enc by base64encode from package base64enc
replace jsonlite base64_enc by base64encode from package base64enc

https://github.com/jeroen/jsonlite/issues/220
kevinykuo/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,rstudio/sparklyr,rstudio/sparklyr