Use an elixir compiled on OTP-20+ for 1.7 images

[NobbZ]

Due to a changed internal debugging format, OTP 21 does not support the OTP-19 style debug info. This causes a lot of errors about not found debug symbols when trying to use dialyzer in this constellation.

To actually make dialyzer happy again one needs to use an elixir that is compiled with OTP 20 at least.

[amuino]

It seems that the problem comes from using the Precompiled release from Github instead of the custom-built versions in repo.hex.pm that were being used for 1.6.x (along with the 1.6.6-otp-21 style tags)

Not sure what is the reason for this change?

[c0b]

it's not a change recently, it was using the Precompiled release since beginning of this repo (early 2016) because @josevadim recommended that way

#87 and a few other threads have discussion we will change to use the repo.hex.pm builds...

[dideler]

Think this can be closed now?