Build Elixir 1.7 for OTP 21

[maennchen]

Build Elixir release manually until hexpm/bob#17 is ready.

Temporary fix for #90.

[c0b]

Looks good

[andresgutgon]

Can this be merged?

[c0b]

right, the hexpm/bob#17 seems won't be ready very soon

while if merge this right now and push to official elixir:v1.7* there might be a shock to those already using elixir:v1.7* images in production, I don't have a number how many of those will be affected, and most probably no impact as well, but need to be careful because when we blindly upgrade elixir:v1.6 image to be from erlang:20 upgrade to erlang:21 that caused some problems to those who trusted to write like FROM elixir:v1.6 as base image
this ALL depends on how different is made by this change, from a min-erlang version Precompiled.zip to a source based builds may be; Let's do some more testing

[maennchen]

@c0b There should be no change for existing users except that dialyzer actually works. Otherwise the images are identical.

[maennchen]

(exactly the same Elixir & Erlang version, this should not affect any users in production)