Prevent unwanted card creation by authenticating requests.

A simple token is enough to ensure that people stumbling across this
service cannot inadvertantly or maliciously create cards in the
associated Trello board.
Use guard condition instead of if/else for auth token.
Fix typo in README.
Remove another Ruby 1.8.7 Gemfile constraint.

This was introduced in 8a0a214b6a150231c2d88a5fda2cfe88b25c293d.

I ran `bundle update ruby-trello` to get the latest version of the gem and its
dependencies.
Fail fast if environment variables are not present.
Extract trello list ID into local variable.