Use MAKE="make -j$(nproc)" to speed up bundle install step

[rubys]

Fixes #149

[olivier-thatch]

@rubys Thanks for making this change, as I never got my PR to pass tests.

However, while this is an improvement over the current state of things, using a numerical value is less flexible than using $(nproc): the latter would automatically scale the number of jobs to the number of processors accessible to the image builder. It would be nice to support a special auto value that calls nproc instead.