Allow for custom builds to be downloaded

I ran into Issue #18.
I managed to build a custom Dart SDK version that would run on Heroku's old glibc library. ( check https://github.com/selkhateeb/heroku-vagrant-dart-build/ for more info on how I did it)

This pull request allows the user to specify a custom build
`heroku config:set DART_SDK_URL=<url to download tar.gz file>`