Specify VM_CHECK_MODE explicitly

In my understanding, `VM_CHECK_MODE` should be Integer and I'm not sure
how `-DVM_CHECK_MODE` would behave. To make the matters simple, let me
pass the mode explicitly.

.travis.yml

@@ -143,7 +143,7 @@ env:
     <<: *make-test-only
     env:
       - GEMS_FOR_TEST=
-      - cppflags='-DRUBY_DEBUG -DVM_CHECK_MODE -DTRANSIENT_HEAP_CHECK_MODE -DRGENGC_CHECK_MODE -DENC_DEBUG'
+      - cppflags='-DRUBY_DEBUG -DVM_CHECK_MODE=1 -DTRANSIENT_HEAP_CHECK_MODE -DRGENGC_CHECK_MODE -DENC_DEBUG'
 
   - &VM_CHECK_MODE
     name: VM_CHECK_MODE=3