upx: slap on an el_capitan MaximumMacOSRequirement

upx 3.91 is completely broken on Sierra, and there's no easy
rescue. Need to wait for the next release, for which no one seems to
have an ETA.

See:
- https://sourceforge.net/p/upx/bugs/248/
- upx/upx#4

Formula/upx.rb

@@ -17,6 +17,10 @@ class Upx < Formula
 
   depends_on "ucl"
 
+  # https://sourceforge.net/p/upx/bugs/248/
+  # https://github.com/upx/upx/issues/4
+  depends_on MaximumMacOSRequirement => :el_capitan
+
   resource "lzma" do
     url "https://downloads.sourceforge.net/project/sevenzip/LZMA%20SDK/lzma938.7z"
     sha256 "721f4f15378e836686483811d7ea1282463e3dec1932722e1010d3102c5c3b20"