By default, via pkg install, in Freebsd the old version of Elixir - 1.4.5 and OTP 19 - are installed. Whereas I want the latest versions - Elixir 1.5 and OTP 20.
Will I have to install them from sources? Has anybody done this? How difficult it was?
Yes. Version managers for various languages have their own repository of versions that can be installed. For example, I use asdf on OSX. OSX does not have a native package manger, but I am still able to install different versions of Elixir and Erlang for the different projects that I work on.