Properly pass a space-separated list of packages to `npm install`

Fixes #490.

nvm.sh

@@ -793,7 +793,7 @@ nvm() {
       local INSTALLS
       INSTALLS=$(nvm use $VERSION > /dev/null && npm list --global --parseable --depth=0 2> /dev/null | tail -n +2 | \grep -o -e '/[^/]*$' | cut -c 2- | \grep -v npm | xargs)
 
-      npm install -g --quiet $INSTALLS
+      echo "$INSTALLS" | xargs npm install -g --quiet
     ;;
     "clear-cache" )
       rm -f $NVM_DIR/v* "$(nvm_version_dir)" 2>/dev/null

test/slow/nvm copy-packages/should work as expected

@@ -8,7 +8,7 @@ nvm use 0.10.28
 
 EXPECTED_PACKAGES="autoprefixer bower david eslint grunt-cli grunth-cli http-server jshint marked node-gyp recursive-blame uglify-js yo"
 
-npm install -g --quiet $EXPECTED_PACKAGES
+echo "$EXPECTED_PACKAGES" | xargs npm install -g --quiet
 
 nvm use 0.10.29
 ORIGINAL_PACKAGES=$(npm list --global --parseable --depth=0 2> /dev/null | tail -n +2 | \grep -o -e '/[^/]*$' | cut -c 2- | \grep -v npm | sort | uniq | xargs)