Restore Swagger-generated files after `make swagger-check` run
Restore Swagger-generated files after `make swagger-check` run

... in order to avoid leaving working copy with files changed in new
lines.

Move from `sh` to `bash` for using `trap`:

* [POSIX spec for `trap` in `sh`
  language](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_28)
  does not specify what `EXIT` condition is.

* [Bash spec for
  `trap`](https://www.gnu.org/software/bash/manual/bashref.html#Bourne-Shell-Builtins)
  specifies what `EXIT` condition is: > If a sigspec is 0 or EXIT, arg
  is executed when the shell exits.

Also instruct `diff` to provide context (`-u`) in order to ease the
Developer in identifying the changes - as now the script when
detecting changes (hence failing) does not any longer keep the
re-generated files in the working copy for inspection.

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch