Merge pull request #424 from buildpacks/platform/0.15

Finalize Platform 0.15

Author: hone

README.md

@@ -52,4 +52,4 @@ These documents currently specify:
 
 - Buildpack API: `0.10`
 - Distribution API: `0.3`
-- Platform API: `0.14`
+- Platform API: `0.15`

platform.md

@@ -99,7 +99,7 @@ Examples of a platform might include:
 
 ## Platform API Version
 
-This document specifies Platform API version `0.14`.
+This document specifies Platform API version `0.15`.
 
 Platform API versions:
  - MUST be in form `<major>.<minor>` or `<major>`, where `<major>` is equivalent to `<major>.0`