try to build 4.0.0-preview.2 in development branch

.travis.yml

@@ -84,6 +84,8 @@ install_linux: &install_linux
   - opam pin add haxe . --no-action
   - opam install haxe --deps-only
   # Build haxe
+  - git checkout 4.0.0-preview.2
+  - git submodule update --init --recursive
   - make package_src -s
   - opam config exec -- make -s STATICLINK=1 libs
   - opam config exec -- make -s -j STATICLINK=1 haxe