Update install directions to prevent sbt deprecation warnings

[danluu]

This is pretty trivial. The directions in the current README.md work correctly, but it causes two sbt deprecation warnings with sbt 0.12.0; the first, for using ~/.sbt instead of ~/project, and the second for using a plugins directory, rather than the project directory, if we do mv ~/.sbt ~/project

[softprops]

It's weird sbt renamed the shared dir. I trust you though :) Thanks for the pull req

[danluu]

Sorry! Your comment made me look at this more closely, and I think you should revert my change. I was testing this by running in sbt in a new term, which opened in my home directory. After trying this elsewhere, it looks like that warning only applies if you have a .sbt directory in your working directory.

I feel a bit silly now :)

[softprops]

No worries. Ill fix it later