Add comment on how to test picrin with Valgrind.

I hope we will really do it on Travis. :-)

Reference: https://github.com/picrin-scheme/picrin/issues/294

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

.travis.yml

@@ -7,12 +7,14 @@ addons:
   apt:
     packages:
       - gcc-multilib
+#     - valgrind
 env:
   - CFLAGS="-m32"
   - CFLAGS="-m64"
 script:
   - perl --version
   - make test
+# - make test-contrib TEST_RUNNER="valgrind -q --leak-check=full --dsymutil=yes --error-exitcode=1 bin/picrin"
   - make clean
   - make debug
   - make test