Making SQuAD to return json (no public API change)

[Ishitori]

Description

Better responsibility separation. _read_data() now just only reads the json into memory, not parses it. It allows to get the json as-is later, if needed. I use it to plug in official evaluation script

Checklist

Essentials

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• _read_data method only reads json file, not parses it