better cedar-14 ruby install error message

Show full stack trace on all unexpected errors
Keep same error interface

Only suppress backtrace from likely errors
travis, again
Merge pull request #309 from heroku/schneems/better-ruby-version-error

better cedar-14 ruby install error message
bump to v128
v128 changelog
Document forking
Record old/new commit SHAs
Document strict asset precompilation
Allow running arbitrary tasks on compile

Required:

    heroku labs:enable user-env-compile
Document custom compile tasks
Remove obsolete mention of user-env-compile
Update README to explain fork.
Fix Tim Pope forked changes lost in merge