that I see on travis CI

Oracle doesn't support enum types

I'm not sure why there is a reference to Oracle in the discussion of https://github.com/h2database/h2database/pull/451. Oracle does not support an enum type (although it can be emulated, as in any database, using a `CHECK` constraint).

* Renamed enum NULL_CONSTRAINT to NullConstraintType
* Replaced some switches with if-statement
* Check for Oracle-Mode when parsing Oracles-SQLs NOT NULL ENABLE
VALIDATE syntax. NOTE: 
* Added DbTypeEnum-class to Mode to have a String-free dbType check
featuring the method Database.isDbTypeOneOf(DbTypeEnum... dbTypes)

Oracles NOT NULL syntax can have optional ENABLE VALIDATE added e.g  
  create table T (C int NOT NULL ENABLE VALIDATE)
Some SQL-generators produce such SQL and H2 can now parse it with this
change.

Issue#647 Removed and ignored generated file.

Issue#628 NPE in CTE