Fix error on non-default postgres schema

[ypcat]

>>> conn = psycopg2.connect(url)
>>> conn.cursor().execute("CREATE SCHEMA foo")
>>> dbSchema = postgres.PostgresSchema(conn)
>>> dbSchema.ensureSchemaTable()
Traceback (most recent call last):
  File ".../schemup/schemup/validator.py", line 23, in findSchemaMismatches
    for tableName, expectedSchema in dbSchema.getVersionedTableSchemas():
  File ".../schemup/schemup/dbs/postgres.py", line 88, in getVersionedTableSchemas
    "SELECT table_name, schema"
psycopg2.ProgrammingError: relation "schemup_tables" does not exist
LINE 1: SELECT table_name, schema FROM schemup_tables WHERE is_curre...

Use CREATE TABLE IF NOT EXISTS to skip checking against information_schema.tables. Requires Postgres 9.1