CRAN check notes

[hadley]

checking R code for possible problems ... NOTE
check_ISO_code: no visible binding for global variable ‘NoAm_country’
check_ISO_code: no visible binding for global variable ‘SoAm_country’
check_ISO_code: no visible binding for global variable ‘Oceana_country’
check_ISO_code: no visible binding for global variable ‘Africa_country’
check_ISO_code: no visible binding for global variable ‘Asia_country’
check_ISO_code: no visible binding for global variable ‘Eur_country’

(discovered during reverse dependency checks for ggplot2)