travis: Use -O3 option (GH-5599)

We don't use debugger on Travis.

.travis.yml

@@ -15,7 +15,8 @@ env:
     - OPENSSL=1.1.0g
     - OPENSSL_DIR="$HOME/multissl/openssl/${OPENSSL}"
     - PATH="${OPENSSL_DIR}/bin:$PATH"
-    - CFLAGS="-I${OPENSSL_DIR}/include"
+    # Use -O3 because we don't use debugger on Travis-CI
+    - CFLAGS="-I${OPENSSL_DIR}/include -O3"
     - LDFLAGS="-L${OPENSSL_DIR}/lib"
     # Set rpath with env var instead of -Wl,-rpath linker flag
     # OpenSSL ignores LDFLAGS when linking bin/openssl