Add MaterializedView relation type, update Snowflake adapter

[adrianisk]

Fix for issue #1430

-- 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 - this looks great!

We're going to cut an 0.13.1 bugfix release relatively soon (within the next ~week) whereas 0.14.0 is going to take a little bit longer (5-6 weeks). If you're interested in getting this live for 0.13.1, you could apply your changed on top of dev/0.13.1 and then change the destination branch for this PR to dev/0.13.1 accordingly. If you're happy waiting for 0.14.0 to use this, then I think we can merge as-is once the automated tests pass

[adrianisk]

Not sure if I can change to source branch so I'm just going to re-do the pr from dev/0.13.1