travis.yml: Fix the getopt problem

The enhanced getopt is now not selected with a configure switch
anymore, but by setting the PATH to the right location.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200923073931.74769-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>