set modules option of es2015 preset to false

[cpsievert]

Thanks to @alandipert for suggesting this change. The motivation is to suppress the "use strict" output which makes ramnathv/htmlwidgets#326 an easier problem to solve.

I'm sure there's an argument to be had for wanting to "use strict", so perhaps we should consider adding an argument to control this in babel_transform()?

[timelyportfolio]

@cpsievert thanks so much, this is great. I'm excited about your work on ramnathv/htmlwidgets#329.