TeX.pm: allow verbatim environment declaration without trailing space

[MayeulC]

Previously, a trailing space was matched by \s+, but this is counter-intuitive, and couldn't make it work before digging in the code.

Match the behavior of non-verbatim environment by making it \s* (allowing, but not requiring trailing spaces).

[mquinson]

Thanks a lot, I'll merge this one as soon as the tests are done.