Fallback on platforms which don't have Process.fork

Spring doesn't work on platforms which don't have Process.fork, so just
shell out to the actual command instead.

This ensures that if you use spring binstubs on a project where one
developer uses Linux and another uses Windows, the Windows developer
will still be able to run (for example) bin/rails without trouble, they
will just not receive the speedup that Spring provides.

Closes #91.
Fix passing of UTF-8 data with OkJson