Use unless instead of 'if !expression'.

In ruby, ! is not only an operator but also a function call. Replacing
it with unless will make our scripts a little bit faster.

Fixes #77.
Undo modification of conditional statements.

By using unless, I accidentally change if !(expression) || ...
to reverse the entire expressions's chain of execution. These
multi-expression conditional statements don't need unless.
Merge use-unless-instead-of-not (#79) into master

Use unless instead of 'if !expression'.

In ruby, ! is not only an operator but also a function call. Replacing
it with unless will make our scripts a little bit faster.

Fixes #77.