Remove ggplot2 aes warnings

[olivroy]

Fix #235.

Approach:

look at whatever compute groups return, and add anything else to dropped aesthetics.

I also changed the limits since the current limits show an empty plot. https://business-science.github.io/tidyquant/reference/geom_bbands.html#ref-examples

[olivroy]

@mdancho84 , whenever you have time, you can merge the 2 pull requests, as it solves errors !