Split RemoveOldStories spec in two

One assertion per test.
Update last_fetched on pruned feeds

This should make sure pruned stories don't reappear
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.
Merge branch 'master' into update-last-fetched-on-prune

Conflicts:
	app/repositories/feed_repository.rb
	spec/repositories/feed_repository_spec.rb
Merge pull request #279 from Koronen/update-last-fetched-on-prune

Update last_fetched on prune