Fix per-version sbt plugin from .sbt support for 0.13.*

Before this changeset, here's the output from a project setup with sbt version 0.13.5
  [info] Loading global plugins from /Users/irfan/.sbt/0.13/plugins/project
  [info] Loading global plugins from /Users/irfan/.sbt/0.13/plugins

With this changeset, the output for the same project
  [info] Loading global plugins from /Users/irfan/.sbt/0.13.5/plugins