Use brew asdk, changes for latest node (#608)

Closes #563 
Closes #587
Closes #600

This makes the following changes:

* Installs asdf via Homebrew, thanks @jdbann in #592 and @gssbzn in #597
* The latest version of the asdf nodejs plugin takes care of the keyring stuff itself.

mac

@@ -148,8 +148,8 @@ brew link --force heroku
 
 fancy_echo "Configuring asdf version manager ..."
 if [ ! -d "$HOME/.asdf" ]; then
-  git clone https://github.com/asdf-vm/asdf.git ~/.asdf --branch v0.5.0
-  append_to_zshrc "source $HOME/.asdf/asdf.sh" 1
+  brew install asdf
+  append_to_zshrc "source $(brew --prefix asdf)/asdf.sh" 1
 fi
 
 alias install_asdf_plugin=add_or_update_asdf_plugin
@@ -165,7 +165,7 @@ add_or_update_asdf_plugin() {
 }
 
 # shellcheck disable=SC1090
-source "$HOME/.asdf/asdf.sh"
+. "$(brew --prefix asdf)/asdf.sh"
 add_or_update_asdf_plugin "ruby" "https://github.com/asdf-vm/asdf-ruby.git"
 add_or_update_asdf_plugin "nodejs" "https://github.com/asdf-vm/asdf-nodejs.git"
 
@@ -187,7 +187,6 @@ number_of_cores=$(sysctl -n hw.ncpu)
 bundle config --global jobs $((number_of_cores - 1))
 
 fancy_echo "Installing latest Node ..."
-bash "$HOME/.asdf/plugins/nodejs/bin/import-release-team-keyring"
 install_asdf_language "nodejs"
 
 if [ -f "$HOME/.laptop.local" ]; then