Revdep e-mail improvements

[krlmlr]

• new variables my_version and you_cant_install
• updated e-mail template, tested with the current DBI revdep check results

NEWS entry:

* Automated revdep check e-mails now can use the new `my_version` and
  `you_cant_install` variables. The e-mail template has been updated
  to use these variables (#1285, @krlmlr).

[krlmlr]

Use explicit version argument, version number at point of release unknown when revdep-checking.

[krlmlr]

Change defaults to send only for problems.

[krlmlr]

Changed defaults to send only for problems, and using explicit version argument. Tested with RSQLite.