travis: Add unix NaN-boxing build to Travis builds.

This will help make sure new commits work with OBJ_REPR_D.

.travis.yml

@@ -17,6 +17,7 @@ script:
   - make -C minimal test
   - make -C unix deplibs CC=gcc-4.7
   - make -C unix CC=gcc-4.7
+  - make -C unix CC=gcc-4.7 nanbox
   - make -C bare-arm
   - make -C qemu-arm test
   - make -C stmhal