persistence/mysql: increase storagesize for coloritem

this is needed because the value that will be stored may increase the 25
characters; e.g. with an RGB Color Picker:

0:43:17.985 ERROR o.o.p.m.i.MysqlPersistenceService[:317]- mySQL: Could not store item 'gsAmbient' in database with statement 'INSERT INTO Item2 (TIME, VALUE) VALUES(NOW(),'217.0854271356784,78.0392156862745,100') ON DUPLICATE KEY UPDATE VALUE='217.0854271356784,78.0392156862745,100';': Data truncation: Data too long for column 'Value' at row 1

Signed-off-by: Manuel Traut <manut@mecka.net>