import Config and test fix

[quentin-bettoum]

import Config instead of use Mix.Config as advised in the warning.

Updated required elixir version from 1.7 to 1.14

Change otp_release from 21.0 to 25.0

And apparently, the TypeError message changed in latest versions of node, thus making a test to fail, so I fixed it.

[quentin-bettoum]

I don't know where to update the node version for the travis build.