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
Fix tests to work on cedar-14 by default
Ensure cache doesn't grow unbounded

Right now when we copy files over we use `cp -a`. This can result in duplicate files as it does not clear the directory when we copy assets:

```
2.1.4  /tmp
$ touch foo/woo
2.1.4  /tmp
$ cp -a foo/. bar
2.1.4  /tmp
$ ls bar
woo
2.1.4  /tmp
$ touch foo/zoo
2.1.4  /tmp
$ cp -a foo/. bar
2.1.4  /tmp
$ ls bar
woo	zoo
```

This behavior is somewhat unintuitive when calling `LanguagePack::Cache#store`. You can see the existing behavior of `LanguagePack::BundlerCache#store` was to clear before storing.

This PR makes the default behavior of store clearing out the target directory before copying over files.

When you want to copy over files in a folder without deleting use `LanguagePack::BundlerCache#add` which does a slightly better job of letting the user know that what they're doing might cause unbounded cache growth.
Merge pull request #314 from heroku/schneems/fix-300

Ensure cache doesn't grow unbounded
bump to v129
v129
Merge branch 'master' of https://github.com/heroku/heroku-buildpack-ruby