Add MaterializedView relation type, update Snowflake adapter

[adrianisk]

Fix for issue #1430, re-doing #1431 on the dev/0.13.1 branch

-- Adds MaterializedView to the list of RelationTypes in the BaseRelation class
-- Updates the snowflake__list_relations_without_caching macro to handle 'MATERIALIZED VIEW' table_type

I didn't find any unit tests for the BaseRelation parsing, so I just tested manually. Let me know if there's a good way to add a test for this.

Prior to the fix running a model in this project returned:

* Deprecation Warning: The adapter function `adapter.already_exists` is deprecated and will be removed in
 a future release of dbt. Please use `adapter.get_relation` instead.
 Documentation for get_relation can be found here:
 https://docs.getdbt.com/reference#adapter

Encountered an error:
Runtime Error
  Invalid arguments passed to "SnowflakeRelation" instance: type.'MATERIALIZED VIEW' is not one of ['table', 'view', 'cte', None]  

After the fix the model runs successfully:

Running with dbt=0.13.0
* Deprecation Warning: The adapter function `adapter.already_exists` is deprecated and will be removed in
 a future release of dbt. Please use `adapter.get_relation` instead.
 Documentation for get_relation can be found here:
 https://docs.getdbt.com/reference#adapter

Found 140 models, 364 tests, 0 archives, 0 analyses, 103 macros, 0 operations, 0 seed files, 0 sources

09:31:09 | Concurrency: 3 threads (target='production_bi')
09:31:09 |
09:31:09 | 1 of 1 START table model bi.the_model................... [RUN]
09:31:17 | 1 of 1 OK created table model bi.the_model.............. [SUCCESS 1 in 8.01s]
09:31:17 |
09:31:17 | Finished running 1 table models in 14.96s.

Completed successfully

Done. PASS=1 ERROR=0 SKIP=0 TOTAL=1

[drewbanin]

Thanks @adriank-convoy! I just kicked off the tests here - we'll be good to merge this for 0.13.1 when they pass :)

[beckjake]

I can't get github to display it, but this passed CI here: https://dev.azure.com/fishtown-analytics/dbt/_build/results?buildId=5

[drewbanin]

@beckjake can you merge this and then work with @cmcarthur to cut an alpha of 0.13.1?