Make SQLiteDB target after requirements installed

I introduced a bug [0][1] which caused `make install` to fail on fresh
installs. This happened because the `DB` target was a prerequisite of
the `install` target, which meant `DB` was trying to reference
requirements, i.e. Django, before they had been installed.

This removes the prerequisite and allows the installation to complete as
intended.

Reported-By: @nedbat

[0] fc044609e2c048b19f37f5086c8cd8a69333ead2
[1] https://github.com/edx/xblock-sdk/pull/28#issuecomment-53821041