Adjust to sbt 0.13

### steps
1. try using sbt-sh with sbt 0.13

### problem

```
[error] /Users/xxx/.sbt/0.13.0/staging/1234/sbt-sh/src/main/scala/org/sbtsh/SbtShPlugin.scala:4: object CommandSupport is not a member of package sbt
[error] import sbt.CommandSupport._
[error]            ^
[error] one error found
[error] ({git://github.com/steppenwells/sbt-sh.git}sbt-sh/compile:compile) Compilation failed
```

### what this changes

It removes the import line.