Update assert-plus to version 1 (and make tests pass on node v7.6)

[jharris4]

No description provided.

[arekinath]

So it looks like the major bump in assert-plus was due to assert.number() now accepting Infinity. I've changed our one usage of it (as assert.optionalNumber()) to asssert.optionalFinite() to deal with this and included that in the version of your change that I've merged as 011fa33.

Thanks!