Change maintainer e-mail

to ensure long-term maintainability because I don't know for how long the old address will be valid. See CRAN Repository Policy:

Explain any change in the maintainer’s email address and if possible send confirmation from the previous address (by a separate email to CRAN-submissions@R-project.org) or explain why it is not possible. If the package needs special treatment (for example if vignettes can only be run or re-built on the maintainer’s machine or take a very long time), say so on the submission form.