fix db migration on postgressql server (fix #173)

In psql :string is mapped to character varying(255), witch is to short for ssh keys, so it shuld be :text.

Also in psql you can not convert int to bool implicit, so th conversion is failing, ruby construct the SELECT Statements with =1 anyway. So if you convert them do boolean by hand, the plug-in crashes because it is trying do compare int to boolean witch is not permitted in psql.