run on latest version on elixir, starting from official docker image

[shdblowers]

Fixes #11

Description

By starting from official docker image, we can now keep up with latest versions of Elixir releases.

Motivation and Context

The current docker image we are using is getting outdated.

How Has This Been Tested?

The tests pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage increased (+3.1%) to 86.822% when pulling 264324e on test_on_latest_elixir into 7d20673 on master.

[coveralls]

Coverage decreased (-25.6%) to 58.14% when pulling 264324e on test_on_latest_elixir into 7d20673 on master.