Bumped  `bignum` dependency version to 0.9.0

I bumped version of `bignum` dependency to version 0.9.0 branch to
allows to use Dulcimer with node version 0.11
Bumped  `nodeunit` dependency version to 0.9.0

I bumped version of `nodeunit` dependency to version 0.9.0 branch to
allows to use Dulcimer with node version 0.11