bump primary package mirror to HTTPS

Makefile

@@ -17,7 +17,7 @@ MXE_TARGETS        := i686-w64-mingw32.static
 
 DEFAULT_MAX_JOBS   := 6
 SOURCEFORGE_MIRROR := downloads.sourceforge.net
-PKG_MIRROR         := http://s3.amazonaws.com/mxe-pkg
+PKG_MIRROR         := https://s3.amazonaws.com/mxe-pkg
 PKG_CDN            := http://d1yihgixbnrglp.cloudfront.net
 GITLAB_BACKUP      := http://gitlab.com/starius/mxe-backup2/raw/master/