Add OPENSSL_INCLUDE_DIR env

.travis.yml

@@ -5,6 +5,7 @@ before_install:
   - export CPPFLAGS="$CPPFLAGS -I/usr/local/opt/openssl/include"
   - export LDFLAGS="$LDFLAGS -L/usr/local/opt/openssl/lib"
   - export OPENSSL_ROOT_DIR="/usr/local/opt"
+  - export OPENSSL_INCLUDE_DIR="/usr/local/opt/openssl/include"
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew outdated openssl || brew upgrade openssl; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew unlink protobuf; fi