$ brew upgrade
Warning: You are using macOS 10.11.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Discourse, Twitter or IRC. You are responsible for resolving any issues you
experience, as you are running this old version.
==> Upgrading 7 outdated packages:
doxygen 1.8.14 -> 1.8.15, ffmpeg 4.0.2 -> 4.1.3, ilmbase 2.2.1 -> 2.3.0, libpng 1.6.36 -> 1.6.37, lz4 1.8.3 -> 1.9.0, opencv 3.4.2 -> 4.1.0_1, openexr 2.2.0_1 -> 2.3.0
==> Upgrading libpng
==> Downloading https://downloads.sourceforge.net/libpng/libpng-1.6.37.tar.xz
==> Downloading from https://jaist.dl.sourceforge.net/project/libpng/libpng16/1.
######################################################################## 100.0%
==> ./configure --disable-silent-rules --prefix=/usr/local/Cellar/libpng/1.6.37
==> make
==> make test
==> make install
? /usr/local/Cellar/libpng/1.6.37: 27 files, 1.2MB, built in 3 minutes 59 seconds
Removing: /usr/local/Cellar/libpng/1.6.36... (28 files, 1.2MB)
Removing: /Users/xxxx/Library/Caches/Homebrew/libpng--1.6.36.tar.xz... (0B)
==> Upgrading ffmpeg
==> Installing dependencies for ffmpeg: aom, frei0r, libtasn1, nettle, libffi, p11-kit, libevent, unbound, gnutls, fribidi, ninja, meson, glib, pixman, cairo, gobject-introspection, graphite2, harfbuzz, libass, libbluray, libsoxr, opencore-amr, doxygen, little-cms2, openjpeg, rtmpdump, flac, libsndfile, libsamplerate, rubberband, speex, autoconf-archive, giflib, webp, leptonica, tesseract and x265
==> Installing ffmpeg dependency: aom
==> Cloning https://aomedia.googlesource.com/aom.git
Cloning into '/Users/xxxx/Library/Caches/Homebrew/aom--git'...
fatal: unable to access 'https://aomedia.googlesource.com/aom.git/': Failed to connect to aomedia.googlesource.com port 443: Operation timed out
Error: An exception occurred within a child process:
DownloadError: Failed to download resource "aom"
Failure while executing; `git clone --branch v1.0.0 https://aomedia.googlesource.com/aom.git /Users/xxxx/Library/Caches/Homebrew/aom--git` exited with 128. Here's the output:
Cloning into '/Users/xxxx/Library/Caches/Homebrew/aom--git'...
fatal: unable to access 'https://aomedia.googlesource.com/aom.git/': Failed to connect to aomedia.googlesource.com port 443: Operation timed out