feat: bump go version to 1.15 since we use URL.RawFragment

go.mod

 module github.com/privacybydesign/irmago
 
-go 1.13
+go 1.15
 
 require (
 	github.com/alexandrevicenzi/go-sse v1.3.1-0.20200117161408-7b23d5ff7420