Add changes before committing in bin/release script

We update the `bin/release` script to add the changes before committing.
Otherwise, we don't commit the version bump made to the README and the
mix.exs file.

bin/release

@@ -8,6 +8,7 @@ release_version="${2}"
 sed -i '' "s/$previous_version/$release_version/" README.md
 sed -i '' "s/$previous_version/$release_version/" mix.exs
 
+git add .
 git commit -m "Release version $release_version"
 git tag "v$release_version"
 git push origin "v$release_version"