heroku-buildpack-ruby

Revert "Bump bundler 1.12.5"

This reverts commit 377d8536b9d2571e9b78b14b90a8dc11da7bca8d.

Caused an issue internal ticket reference 364182

Gemfile:

```
ruby "2.3.0"
```

Gemfile.lock

```
RUBY VERSION
   ruby 2.3.0p0

BUNDLED WITH
   1.12.4
```

Results in an invalid download URL

```
 !     An error occurred while installing Ruby ruby-2.3.0-p0
 !     For supported Ruby versions see https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
 !     Note: Only the most recent version of Ruby 2.1 is supported on Cedar-14
 !     Command: 'set -o pipefail; curl -L --fail --retry 3 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/cedar-14/ruby-2.3.0-p0.tgz -s -o - | tar zxf - ' failed unexpectedly:
 !
 !     gzip: stdin: unexpected end of file
 !     tar: Child returned status 1
 !     tar: Error is not recoverable: exiting now
```

The URL should be:

```
https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/cedar-14/ruby-2.3.0.tgz
```

Without the p0.

Investigating