Success threshold should be greater_than_or_equal

[alanman]

!roll 5d10 >> 6

When I roll this, I read it as roll 5d6, difficulty 6.
RPBot reads it as 5d6, difficulty 7

My understanding is the difficulty should be greater than or equal, not just greater than.

Same goes for the other difficulty measure: !roll 1d20 > 10 will grant success at 11 or higher instead of at the expected 10.

For better logic and understanding, I request this change be made.

[alanman]

Submitted a pull request with the changes.
#11