Set minimum Sass precision to 8

[tagliala]

Refers to #2276, #2277

Bootstrap requires a minimum sass precision of 8

This is causing some issues on button height. In example, if you try to append buttons to form controls the result will be 1px wrong

I'm not an English native speaker, so I'm not confident in submitting a PR to the readme. I think a copy&paste from bootstrap's docs with some minor edits should be enough

[linuxonrails]

+1

[mshibuya]

Default precision is 10 now, instead of 5.
sass/sass#2297