.travis.yml: Add Perl5.20

Signed-off-by: Niels Thykier <niels@thykier.net>
Fix O_CREAT without O_TRUNC in utf8_open.t

Signed-off-by: Niels Thykier <niels@thykier.net>
Fatal: Move up a variable to enable more reuse

Signed-off-by: Niels Thykier <niels@thykier.net>
Fatal: Remove a redundant if

Both paths basically compute the same end result.

Signed-off-by: Niels Thykier <niels@thykier.net>
Fatal: Rewrite/Reorder some comments

Signed-off-by: Niels Thykier <niels@thykier.net>
autodie: Mention that some builtins have prototype issues

Mention that e.g. chdir(BAREWORD) does not work (as usual) under
autodie due to differences in built-in behaviour vs. the prototype
Perl provides to us.

Signed-off-by: Niels Thykier <niels@thykier.net>