In postgresql, how can I return a boolean value instead of string on a jsonb key?

Simply cast a text to boolean:

create table jsonb_test (id int, data jsonb);
insert into jsonb_test values
(1, '{"is_boolean" : true}'),
(2, '{"is_boolean" : false}');

select id, data, (data->>'is_boolean')::boolean as is_boolean
from jsonb_test
where (data->>'is_boolean')::boolean

 id |          data          | is_boolean 
----+------------------------+------------
  1 | {"is_boolean": true}   | t
(1 row)

Note that you can also cast other json text values to boolean, examples:

insert into jsonb_test values
(3, '{"is_boolean" : "true"}'),
(4, '{"is_boolean" : "false"}'),
(5, '{"is_boolean" : "t"}'),
(6, '{"is_boolean" : "f"}'),
(7, '{"is_boolean" : "on"}'),
(8, '{"is_boolean" : "off"}');

select id, data, (data->>'is_boolean')::boolean as is_boolean
from jsonb_test
where (data->>'is_boolean')::boolean

 id |          data          | is_boolean 
----+------------------------+------------
  1 | {"is_boolean": true}   | t
  3 | {"is_boolean": "true"} | t
  5 | {"is_boolean": "t"}    | t
  7 | {"is_boolean": "on"}   | t
(4 rows)

Read about valid literals for boolean type in the documentation.

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Update

Postgres 11 adds casts from JSONB scalars to numeric and boolean data types. This query will work only for regular boolean JSONB scalars (i.e. true or false):

select id, data, (data->'is_boolean')::boolean as is_boolean
from jsonb_test
where (data->'is_boolean')::boolean