restrict rxjs to version 6.3.x (6.4 requires a newer typescript version)
restrict rxjs to version 6.3.x (6.4 requires a newer typescript version)

Signed-off-by: Andrew Whitehead <f7337adc4e885e6dd038c60d80945a2799b03f27@gmail.com>

WadeBarnes/TheOrgBook,swcurran/TheOrgBook,WadeBarnes/TheOrgBook,WadeBarnes/TheOrgBook,WadeBarnes/TheOrgBook,swcurran/TheOrgBook,swcurran/TheOrgBook,swcurran/TheOrgBook,WadeBarnes/TheOrgBook,swcurran/TheOrgBook