Small type in test database name

NOTE:
':memory;' should be ':memory:', I remove NAME as django will be
automatically use in-memory if there is no NAME specified.

[ci skip]
Add output for recommends_precompute command

NOTE:
Even through logger has logged most of things, I think it maybe good to have some
output for user when they run recommends_precompute command

[ci skip]
Add use_raw_id option for similarities and recommendations

1. Add use_raw_id option (default = False) to all storages
2. Add test for returned raw id list

[ci skip]
Code formart according to flake8

1. format code
[ci skip]
travis settings update

1. Change pip install order, should install django at last
2. Add flake8 to check code style