Remove support for very old browsers in autoprefixer

These browsers are very old, we don't support them, and, combined, their
usage is about 0.1%. We're still supporting IE 11 through the "last 1
version" option and many versions of Android and ios through the "last 7
years" option.

Thanks to this change, we no longer generate prefixes in flexbox
properties, making debugging in the browser easier. There are no
significant differences in the size of the compiled CSS in production
environments, though (with these changes, the file is about 2% smaller).

app/assets/stylesheets/.browserslistrc

@@ -1,5 +1,2 @@
-last 2 versions
+last 1 version
 last 7 years
-ie >= 9
-Android >= 2.3
-ios >= 7