Introduce notFalse()

[simPod]

I was rewriting this code

/** @var SplFileInfo $file */
$filename = $file->getRealPath();
assert($filename !== false);

into Assertion and the Assertion::notFalse() method was missing. Using Assertion::true($filename !== false) IMO loses the benefit of using Assertion as nice readable wrappers around simple conditions.

notFalse() is similar to notNull(). Especially some PHP internal functions return type of <result>|bool but actually bool there is false -> <result>|false which stands for failure. This is handy instrument how to handle it.

[stefanfisk]

I'd very much appreciate this addition! Would it be accepted if I supplied an update to date PR?