replace table_exists? with data_source_exists?

[isaiah]

Since Rails5, table_exists? only works for tables, which makes acts_as_geolocated fail on materialized views.

http://guides.rubyonrails.org/5_0_release_notes.html#active-record-deprecations

[diogob]

Thanks for the PR @isaiah

Could you add a test case that would fail in the current code base? Creating a materialized view in the spec_helper should be quite straightforward.