refactor: box id as character only

[ijlyttle]

This is a followon from #80.

The only reason we are using the bit64 package is for this bit of code:

# Validate ids supplied
box_id <- function(x) {
  if (!is.null(x) && any(is.na(bit64::as.integer64(x)))) 
    stop("box.com API ids must be (coercible to) 64-bit integers")
  if (!is.null(x))
    return(as.character(bit64::as.integer64(x)))
}

I think we could drop {bit64} by replacing this validator using a regular-expression that checks for digits.

[ijlyttle]

fixed in #264