Warn Java 6 users on sbt startup that compiling answers will fail.

Compiling in Java 6, you get this error:

```
[error] …/fpinscala/answers/src/main/scala/fpinscala/iomonad/Future.scala:122: not found: type AsynchronousFileChannel
[error]   def read(file: AsynchronousFileChannel,
[error]                  ^
[error] one error found
[error] (answers/compile:compile) Compilation failed
```

This adds the following message on sbt startup.

```
[info] Set current project to fpinscala (in build file:…/fpinscala/)
[info] WARNING: JSR-203 "NIO.2" (java.nio.channels.AsynchronousFileChannel) not found.
[info] You are probably running Java < 1.7; answers will not compile.
[info] You seem to be running 1.6.0_27.
[info] Try `project exercises' before compile, or upgrading your JDK.
> █
```

Tested in Java (6, 7) × sbt (0.12.4, 0.13.0).