pkgr: use main branch and Go 1.15 for buildpack (#6273)

.pkgr.yml

@@ -33,4 +33,4 @@ before:
 after:
   - mv bin/gogs gogs
 after_install: ./.packager/hooks/postinst
-buildpack: https://github.com/heroku/heroku-buildpack-go.git
+buildpack: https://github.com/heroku/heroku-buildpack-go.git#main

go.mod

@@ -71,5 +71,5 @@ require (
 	xorm.io/xorm v0.8.0
 )
 
-// +heroku goVersion go1.14
+// +heroku goVersion go1.15
 // +heroku install ./