Both are correct.
The older romanization of the sound for ”し” is "si" but the newer romanization is "shi" which accounts for the fact the actual sound is more "shi" like. The older romanization tried to keep all the sounds in a "two letter" format.
You will still see the older romanization in older text and some advertising and when people who learned their romaji ages ago write in it.
Watch the show opener and you'll see that the creators *spell* it "kasimasi".
Most people writing about the show have been trained in the newer romaji and will spell it "kashimashi".
To be anal.. the correct spelling is to use the hiragana, of course: かしまし。
Seven Seas Entertainment has licensed the manga (see
www.gomanga.com) and chosen to spell it "Kashmashi"