I use this to install Elixir on Codeship.

install_ex.sh

#!/bin/sh

# Forked from https://gist.github.com/jamesotron/44f8962cddef781ab830
# wget https://gist.githubusercontent.com/evax/493c134b7c6711b499f0/raw/6d4aaf11b9f041cd77bc273691579d0fe7f72c9c/install_ex.sh
# . ~/install_ex.sh

# You can override your Elixir and Erlang versions from your shell when you call ./install_ex.sh.
ERLANG_VERSION=${ERLANG_VERSION:-18.3}
ELIXIR_VERSION=${ELIXIR_VERSION:-1.2.3}

# Erlang
echo Installing Erlang version ${ERLANG_VERSION}
wget "https://s3.amazonaws.com/heroku-buildpack-elixir/erlang/cedar-14/OTP-${ERLANG_VERSION}.tar.gz"
tar zxf OTP-${ERLANG_VERSION}.tar.gz
ERL_PATH=$(realpath ~/OTP-${ERLANG_VERSION})
${ERL_PATH}/Install -minimal ${ERL_PATH}
export PATH="${ERL_PATH}/bin:$PATH"

# Elixir
echo Installing Elixir version ${ELIXIR_VERSION}
wget "https://s3.amazonaws.com/s3.hex.pm/builds/elixir/v${ELIXIR_VERSION}.zip"
mkdir ~/elixir-${ELIXIR_VERSION}
EX_PATH=$(realpath ~/elixir-${ELIXIR_VERSION})
cd ${EX_PATH}
unzip -q ../v${ELIXIR_VERSION}.zip
export PATH="${EX_PATH}/bin:$PATH"

mix local.hex --force
mix local.rebar