Express that `apps` depend on `bc`

Look at `kernel/src/ling_%.erl` in `apps/Makefile`.
Complete expressing deps of `railing` - depending on `apps`

See https://github.com/cloudozer/ling/blob/baa355b2d0c3fda00af7c6e384989d8fb4db8013/railing/escriptize#L38

This commit completes 2b45bb8.
Now that deps among components are clear, set default target to railing

All other targets will be built as needed.

Also simplify Travis CI config: skip creation of empty image
considering that test.img is built.
Merge pull request #30 from lucafavatella/simplify-makefile

Simplify Makefile and Travis CI