Internal Server Error using Postgres database
# Description
Using a Postgres logging database the application fails with an Internal Server Error. SQLAlchemy is trying to create table fields with types that do not exist in Postgres.

# Environment

- operating system: Ubuntu 16.04
- Python version: Python 2.7
- PyWPS version: 4.0.0
- source/distribution
 - [x] git clone
 - [ ] Debian
 - [ ] PyPI
 - [ ] zip/tar.gz
 - [ ] other (please specify): 
- web server
 - [ ] Apache/mod_wsgi
 - [ ] CGI
 - [x] other (please specify): Flask

# Steps to Reproduce
Happens with any request. Db connection string:
database=postgresql://pywps:toto123@localhost:5432/pywps-demo

# Additional Information

diff --git a/pywps/dblog.py b/pywps/dblog.py
index 175ec14..cb5a32e 100644
--- a/pywps/dblog.py
+++ b/pywps/dblog.py
@@ -21,7 +21,7 @@ import os
 
 import sqlalchemy
 from sqlalchemy.ext.declarative import declarative_base
-from sqlalchemy import Column, Integer, String, VARCHAR, Float, DateTime, BLOB
+from sqlalchemy import Column, Integer, String, VARCHAR, Float, DateTime, LargeBinary
 from sqlalchemy.orm import sessionmaker
 
 LOGGER = logging.getLogger('PYWPS')
@@ -54,7 +54,7 @@ class RequestInstance(Base):
     __tablename__ = '{}stored_requests'.format(_tableprefix)
 
     uuid = Column(VARCHAR(255), primary_key=True, nullable=False)
-    request = Column(BLOB, nullable=False)
+    request = Column(LargeBinary, nullable=False)
 
 
 def log_request(uuid, request):
['pywps/dblog.py']