Remove build/modules/ at beginning of `grunt` and `grunt test`.

Little-known fact: instead of writing copies of compiled module files to
build/modules/, the bin/jsx-internal script actually just makes hard links
to the master versions of files in the .module-cache/, so re-populating
build/modules/ is very inexpensive.

Closes #856.
Merge pull request #980 from benjamn/issue-856-delete-build-modules

Remove build/modules/ at beginning of `grunt` and `grunt test`.