make server: Don't fire the browser until jekyll is ready

Currently when you `make server`, your browser immediately opens to a dead page, and you must ALT+TAB back to the terminal to know when you can refresh. This PR fixes that:

    ruby -e 'sleep 3; 1.upto(30) { break if system("curl -I -s $(LOCAL)"); print "."; sleep 1 }; exec "$(OPEN)"'
                      ^            ^                                     ^                     ^ ^             ^
                      |            '- -  break when the site is ready - -'                     | '- open site -'
                      |                                                                        |
                      '- - - - - - - - - - -   loop for up to 30 seconds  - - - - - - - - - - -'