Rename {ensure => validate}_length_of
Deprecate ensure_length_of for validate_length_of
Rearrange some things in NEWS
Fix another warning
Tweak allow_value failure message again

If no message is set and the matcher failed it formerly produced this
message:

    Expected errors  when attr is set to "some value"...

Now it produces this message:

    Expected errors when attr is set to "some value"...
Clean up acceptance tests a bit