Update influxdb.R

The call to GET currently returns 'Error in function (type, msg, asError = TRUE)  : couldn't connect to host'.
I traced this to the named arguments to GET. One of the lower level functions is looking for the field 'hostname', not host.
After I renamed the arg in GET to hostname = host, the call to influx worked..I am using httr 0.3, based on help.
Merge pull request #2 from julianrcook/patch-1

Update influxdb.R