fix(migrations): change the order of queries for mysql

Signed-off-by: Roman Dmytrenko <rdmytrenko@gmail.com>

config/migrations/mysql/5_namespaces_relationships.up.sql

@@ -37,12 +37,12 @@ ALTER TABLE segments ADD COLUMN namespace_key VARCHAR(255) NOT NULL DEFAULT 'def
 -- Drop previously created unique index and add a new unique index on namespace_key and key columns
 ALTER TABLE segments DROP INDEX `key`, ADD INDEX `key` (`key`) USING BTREE;
 
--- Add foreign key constraint on namespace_key column referencing key column of namespaces table
-ALTER TABLE segments ADD FOREIGN KEY (namespace_key) REFERENCES namespaces(`key`) ON DELETE CASCADE;
-
 -- Drop primary key constraint and add a new composite primary key on namespace_key and key columns
 ALTER TABLE segments DROP PRIMARY KEY, ADD PRIMARY KEY (`namespace_key`, `key`);
 
+-- Add foreign key constraint on namespace_key column referencing key column of namespaces table
+ALTER TABLE segments ADD FOREIGN KEY (namespace_key) REFERENCES namespaces(`key`) ON DELETE CASCADE;
+
 -- Constraints
 
 -- Add column namespace_key with a default value