update composer.json, add @dev to require-dev section

[gesinn-it-ilm]

After PR #884 has been reverted, and dev-master removed from require section in composer.json, test coverage has dropped significantly for MW 1.40. In codecov it could be checked, from 45,50 % to only 7 %.

After logs are checked for the MW 1.40 in CI, althought it is green light, it is not the real picture. Support for MW 1.40 is failing, and definitely do not use dev-master, but latest version of SMW which is 4.2.0.
This is the error which indicates that:

Error: Call to undefined method BacklinkCache::clear()

This error is fixed on master, so it is clear that it still uses 4.2.0 SMW version.

Fix support for MW 1.40, possible solution is adding "mediawiki/semantic-media-wiki": "@dev" to require-dev section in composer.json.

[gesinn-it-ilm]

"mediawiki/semantic-media-wiki": "@dev" added to composer.json.
Now, CI is working as it should.
Codecov upload works good as well.
PR will be opened.