ci: set PKG_CONFIG_PATH on travis

Homebrew's formula for openssl is "keg-only", which means it does not
install it into /usr/local.  On macOS builds, we need to set
PKG_CONFIG_PATH to include it.

(cherry picked from commit abf53363)

.travis.yml

@@ -17,6 +17,7 @@ env:
   - secure: "YnhS+8n6B+uoyaYfaJ3Lei7cSJqHDPiKJCKFIF2c87YDfmCvAJke8QtE7IzjYDs7UFkTCM4ox+ph2bERUrxZbSCyEkHdjIZpKuMJfYWja/jgMqTMxdyOH9y8JLFbZsSXDIXDwqBlC6vVyl1fP90M35wuWcNTs6tctfVWVofEFbs="
   - GITTEST_INVASIVE_FS_SIZE=1
   - SKIP_APT=1
+  - PKG_CONFIG_PATH=/usr/local/opt/openssl/lib/pkgconfig
  matrix:
   - CMAKE_OPTIONS="-DTHREADSAFE=ON -DENABLE_TRACE=ON -DCMAKE_BUILD_TYPE=Release"
   - CMAKE_OPTIONS="-DTHREADSAFE=OFF -DBUILD_EXAMPLES=ON"