Exec start-app.sh to avoid orphans

When foreman signals `start-app.sh`, children do not receive the
message. Instead the bash process running `start-app.sh` exits and
the subprocess becomes an orphan, getting reparented to init. The result
is that servers don't get stopped correctly and have to be killed
manually which is an error prone process.

This bug is related to alphagov/pp-development#48 and the fix in this
commit is necessary but not sufficient to close it.