Compatibility with iRuby

[translunar]

I played around with making plotrb work with iruby all day today, but no luck so far.

Vincent already works with iPython, so it should be a simple matter to port over support from Vincent into plotrb.

Note that the D3 js file changed in the last few months, which made it a lot more complicated to include it. One clue is here: https://github.com/tristanz/vincent/commit/574d0007c161c99345eb33f94f93d665bf669cc5

[minad]

See https://github.com/SciRuby/plotrb/pull/20