This version is available in the 'devel' branch of Bioconductor
BiocInstaller::useDevel()
biocLite("ENmix")
(this will update a number of packages, but is the recommended solution).
Installing packages from github can also be accomplished with BiocInstaller::biocLite("Bioconductor-mirror/ENmix") or devtools::install_github().
If you install directly from github and are not using the devel branch of Bioconductor, then you will mix package versions and will be asking here, in the not too distant future, why some package fails to load or install with some cryptic error message about unexported symbols or similar. The solution will be to follow the advice of BiocInstaller::biocValid() (downgrade packages to the release version) or to useDevel() and update packages. Either way, you're further ahead in the long run by using the version of Bioconductor needed for the features you wish to use.