Update command that installs 'rbenv' to use 'curl' instead of 'wget'

The 'wget' command was failing for me with this error:

    ERROR: certificate common name `*.a.ssl.fastly.net' doesn't match requested host name `raw.github.com'.

But 'curl' works with the same https URL. Also, the web instructions on
  https://github.com/fesplugas/rbenv-installer
suggest using 'curl'.