Reverse: Limit the trigger word to "reverse text".

This is triggering very often, and it's probably best to limit the trigger word to "reverse text". It triggers with:

- "reverse dns 8.8.8.8."
- "reverse parameter sniffing"
- 'put car in reverse'
t/Reverse.t: Update tests.
Merge pull request #533 from duckduckgo/jag/reverse-limit

Reverse: Limit the trigger word to "reverse text".