This is a new change in RVM, you can make it work with:
rvm --verify-downloads 1 pkg install zlib
A better solution would be to add md5 / sha512 for zlib so that it can be checked
Command parsing has already been fixed, udate rvm, to make it work:
rvm get head
Failure Warning:
rvm pkgoutdated by autolibs :
rvm get stable
rvm autolibs enable
rvm install ruby
RVM , - OSX RVM , homebrew, fink, .