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.

.travis.yml

@@ -19,6 +19,7 @@ env:
   - MBEDTLS_DIR=/tmp/mbedtls
   - SKIP_APT=1
   - SKIP_MBEDTLS_INSTALL=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"