61 update linux installation instruction

[dkuku]

fixes #61
I updated the readme using the latest available versions of Elixir, Erlang and node.js - I had to update ecto to 4.0 because it was throwing errors with older versions, all test are passing.
I'm using asdf package manager for that - I use it locally on my computer and I think it's easy to set up: just 1 package manager for all your needs, instruction is universal, should work on linux and macos.
Heve a look and tell me what you think

[mattwr18]

great job @dkuku I am using your install instructions as we speak, and I'm also a fan of asdf

[mattwr18]

Note that when trying to run apt-get install gpg, I am getting this error:

Package gpg is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gpg' has no installation candidate