Remove blanket from package.json

Blanket has been replaced by istanbul on CI, so it makes less sense to use
blanket here when all it does is add a format warning to every npm command.
Update tests to use .type() instead of .a()

should.js removed this use of #a and #an in 2.0.0 and replaced with #type().
The version being used now is 2.1.1.
Update devDependencies to guard against breaking changes