.travis.yml: Improve ccache use

Per https://ccache.dev/manual/latest.html:

  By default, ccache tries to give as few false cache hits as
  possible. However, in certain situations it’s possible that
  you know things that ccache can’t take for granted.

  [The CCACHE_SLOPINESS environment variable] makes it possible
  to tell ccache to relax some checks in order to increase the
  hit rate.

We can relax the ctime/mtime header checks:

  - include_file_ctime

    By default, ccache also will not cache a file if it
    includes a header whose ctime is too new. This option
    disables that check.

  - include_file_mtime

    By default, ccache will not cache a file if it includes
    a header whose mtime is too new. This option disables
    that check.

We also add a call to clear the cache statistics before running
the build, and display them when the build finishes.

See https://docs.travis-ci.com/user/caching/#ccache-cache



Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>