Use sqlite instead of postgresql on ppc64le builds.
Use sqlite instead of postgresql on ppc64le builds.

There is some problem with postgres on ppc64le, example travis build log:
```
$ psql -c 'create database dirtyfields_test;' -U postgres
psql: could not connect to server: No such file or directory
	Is the server running locally and accepting
	connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?
The command "psql -c 'create database dirtyfields_test;' -U postgres" failed and exited with 2 during .
```

romgar/django-dirtyfields,smn/django-dirtyfields