Travis: fix broken migration testing command
Travis: fix broken migration testing command

It was using this command:
    ./manage.py makemigrations --dry-run -e
which doesn't recognize `-e` parameter. That parameter was changed to
`--check` and the command now works.

This fixes #1451.

swcarpentry/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy