[LMA regression] Runs randomly fail with "SchemaError"s
### Describe the bug
If you run dbt commands enough times, you will eventually get a weird jsonschema error:

```
  None is not of type 'object', 'boolean'

  Failed validating 'type' in metaschema['properties']['definitions']['additionalProperties']:
      {'$id': 'http://json-schema.org/draft-07/schema#',
  <snipped a lot of json here>
  On schema['definitions']['Policy']:
      None
```

That last line is one of a few possible values. It only seems to happen with multiple threads. I assume it involves some aspect of the stuff hologram does for schema generation/caching.

dbt should either wrap `hologram` and retry, or we should fix `hologram` and rely on the fixed version.

### Steps To Reproduce
1) Clone jaffle shop, or any other working dbt project
2) echo ''; while [[ $? -eq 0 ]]; do dbt docs generate --threads=8; done
3) wait a bit

There's possibly more to this, I think I've also triggered it with `dbt run`, but that seems much more difficult.


### Expected behavior
dbt should pass/should not raise big errors about metaschema junk

### Screenshots and log output
See above

### System information
**Which database are you using dbt with?**
- [X] postgres

**The output of `dbt --version`:**
```
installed version: 0.15.0-b2
   latest version: 0.14.3

Your version of dbt is ahead of the latest release!
```

**The operating system you're using:**
Macos Mojave, also seen on docker

**The output of `python --version`:**
Python 3.6.5

### Additional context

diff --git a/core/setup.py b/core/setup.py
index 1186b9e7e..6509e481b 100644
--- a/core/setup.py
+++ b/core/setup.py
@@ -56,7 +56,7 @@ setup(
         'json-rpc>=1.12,<2',
         'werkzeug>=0.15,<0.17',
         'dataclasses==0.6;python_version<"3.7"',
-        'hologram==0.0.4',
+        'hologram==0.0.5',
         'logbook>=1.5,<1.6',
         'pytest-logbook>=1.2.0,<1.3',
         'typing-extensions>=3.7.4,<3.8',
['core/setup.py']