make use of 2 cores in Travis CI

(cherry picked from commit 283ba78399a565cb24d1ddfe73ed596792648383)

.travis.yml

@@ -86,5 +86,5 @@ script:
   - if [ "$STATIC_CHECKS" = "yes" ]; then
       sh ./misc/travis/clang-format.sh;
     else
-      scons platform=$GODOT_TARGET progress=no verbose=yes CXX=$CXX openssl=builtin;
+      scons -j 2 platform=$GODOT_TARGET progress=no verbose=yes CXX=$CXX builtin_openssl=yes;
     fi