Update to hard-code "2.10" in library dependency

The README previously suggested using

    "fi.akisaarinen" %% "rillit" % "0.1.0"

That works with Scala 2.10, but not with Scala 2.11. It might be desirable to change the README as follows, so that it works with both 2.10 and 2.11:

    "fi.akisaarinen" % "rillit_2.10" % "0.1.0"