world map 03 es2015 + update code style

this iteration converts the code to ES2015 using lebab

➜  03 git:(master) ✗ lebab es5.js -o es6.js --transform arrow
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform for-of
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform arg-spread
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform obj-method
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform obj-shorthand
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform multi-var
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform let
➜  03 git:(master) ✗ lebab es6.js -o es6.js --transform template

do check out the other examples in this world map series:

world map 00 original example
world map 01 fix tooltip value
world map 02 d3 v4
world map 03 es2015 + update code style
world map 04 manual breaks + threshold scale
world map 05 linear breaks + quantize scale
world map 06 linear breaks + quantiles scale
world map 07 Jenks natural breaks
world map 08 ckmeans cluster max breaks
world map 09 ckmeans cluster min breaks