Changed \s pattern in sed regex pattern to [[:space:]]. 

erlang.mk uses <code>\s</code> to match whitespace when matching the modules tuple in <code>src/$(PROJECT).app.src</code>. This is a GNU-specific sed extension. This breaks the build with BSD sed implementations. This change moves to <code>[[:space:]]</code> for a POSIX-compliant whitespace pattern. To test this patch, I used both sed and gsed on OS Mavericks to build farwest and it worked. I grabbed gsed from MacPorts.

This should make the sed pattern portable across GNU and BSD implementations.