[INSTALLATION] updated PyYAML version restriction
[INSTALLATION] updated PyYAML version restriction

PyYAML was restricted to `<3.13` which is incompatible with Python 3.7.
This patch update the restriction to `<4.0`.

superdesk/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,ioanpocol/superdesk-core,mdhaman/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,ioanpocol/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,mdhaman/superdesk-core,mdhaman/superdesk-core,ioanpocol/superdesk-core