Fix bin/dotc checkjar of wrong project

[felixmulder]

Noticed that bin/dotc wasn't working for the "Getting Started" instructions anymore. The failing line was:

checkjar $INTERFACES_JAR interfaces/package interfaces

Which generated the error:

[Projects/dotty master] ./bin/dotc tests/pos/HelloWorld.scala
The script is going to build the required jar file /Users/fixel/Projects/dotty/bin/../interfaces/target/dotty-interfaces-0.1-SNAPSHOT.jar by running "sbt interfaces/package"
[info] Loading global plugins from /Users/fixel/.sbt/0.13/plugins
[info] Loading project definition from /Users/fixel/Projects/dotty/project
[info] Set current project to dotty (in build file:/Users/fixel/Projects/dotty/)
[error] Expected ID character
[error] Not a valid command: interfaces
[error] Expected project ID
[error] Expected configuration
[error] Expected ':' (if selecting a configuration)
[error] Expected key
[error] Not a valid key: interfaces (similar: signedArtifacts, signed-artifacts)
[error] interfaces/package
[error]           ^
/Users/fixel/Projects/dotty
The required jar file has not been built by sbt. Please run "sbt interfaces/package"

review: @smarter

[smarter]

Thanks! Merging because the sole test failure is due to an out of memory error.