Make SQLiteDB target after requirements installed

I introduced a bug [0][1] which caused `make install` to fail on fresh
installs. This happened because `make` attempted to create the database
_before_ installing the requisite Python packages, namely Django.

Python packages must now be installed _before_ creating the database.

Reported-By: Ned Batchelder <ned@edx.org>

[0] fc044609e2c048b19f37f5086c8cd8a69333ead2
[1] https://github.com/edx/xblock-sdk/pull/28#issuecomment-53821041
Merge pull request #30 from stvstnfrd/make/fix

Make SQLiteDB target after requirements installed