Added ssl certfile on osx for openssl pacakge on homebrew

A certfile was already specified for osx but only with MacPorts,
this patch adds the certfile given with the package `openssl` with
homebrew.

You can get more info with the command `brew info openssl` on osx with
homebrew and openssl installed.

Signed-off-by: Philippe Loctaux <loctauxphilippe@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/utils/distro.py

@@ -18,6 +18,8 @@ __DEF_OS_LOCATIONS = {
     'darwin': [
       # MacPorts, port curl-ca-bundle
       '/opt/local/share/curl/curl-ca-bundle.crt',
+      # homebrew, package openssl
+      '/usr/local/etc/openssl/cert.pem',
     ],
     'linux-ubuntu': '/etc/ssl/certs/ca-certificates.crt',
     'linux-debian': '/etc/ssl/certs/ca-certificates.crt',