This should give a significant boost to Travis speeds, because most of the
compile time is spent building the toolchain over and over again.
However, the toolchain (or libc or libm) changes only rarely,
so most rebuilds can skip this step.
The hashing has been put into a separate file to keep it
as decoupled as possible from BuiltIt.sh.
lint-shell-scripts searches over the repository looking for shell
scripts. On those found, shellcheck is run against them. If any linting
fails print those warnings and exit with a non-zero exit code.
Run this script automatically in Travis.
It seems like the newest Ubuntu supported by Travis is 16.04. The bundled
gcc is unable to compile the trendy and modern Serenity code, so let's try
to install a newer GCC on the CI bot. :^)
