Use "mail" instead of "Mail" as the default nightly testing mailer.

On Ubuntu "Mail" is not provided. On all existing test systems both "mail" and
"Mail" are provided and are the same. Using "mail" as the default will allow
ubuntu testing to use the default instead of having to set CHPL_MAILER.
Merge pull request #798 from thomasvandoren/update-default-mailer

Use "mail" instead of "Mail" as the default nightly testing mailer.

On Ubuntu "Mail" is not provided. On all existing test systems both "mail" and
"Mail" are provided and are the same. Using "mail" as the default will allow
Ubuntu testing to use the default instead of having to set CHPL_MAILER.