The future2.15 package is a convenience package for those stuck with R versions lower than 2.15 but want the functions introduced in 2.15. If you are using a version of R that is older than 2.15 you would probably want to install the other 'future' packages as well to get the functions provided in those releases of R as well.
Currently there isn't a release on CRAN.
You can, however, download the zip ball or tar ball, decompress and run R CMD INSTALL on it, or use the devtools package to install the development version:
## Make sure your current packages are up to date
update.packages()
## devtools is required
library(devtools)
install_github("future2.15", "Dasonk")
## You probably should install the other future
## packages which provide the functions for newer version of R too
install_github("future3.0", "Dasonk")
# The 'future' package loads the other packages so you don't need
# to remember to load all of them at once.
install_github("future")