Split RemoveOldStories spec in two

One assertion per test.
Update last_fetched on pruned feeds

This should make sure pruned stories don't reappear
Adding tests
Remove trailing whitespace
Don't update last_fetched when given an older timestamp
Extract timestamp validation into a class method
Invert logic for better readability

Avoids a double negation ("unless is_invalid..."), so should be easier to read.