Removed redundant this.nav declaration (ionic-team#6244)

`this.nav = nav` is already present due to the `public nav: NavController` declaration, having it in the constructor as well is redundant

chore(sass): fix Sass linter errors

chore(package): update README

chore(ionic): release beta 2 and update changelog

chore: release ionic beta.1

chore(): beta release 🎉

chore(release): release alpha57

test(menu): fix snapshot test

chore() release alpha.55

chore(): release alpha.54