upgrade to sbt 0.13.7

nothing must slow the ceaseless march into modernity!

safest to hang onto the blank lines in our *.sbt files for now,
though
Merge pull request #20 from NetLogo/sbt-0.13.7

upgrade to sbt 0.13.7
update .gitignore for ENSIME
Seal InputBoxType
Limit Chooseable options to string, double, boolean, logolist
Textbox escapes and unescapes display string on parsing and writing
Allows chooser to have a default value of a string containing "//"
Represent widget constraints as data objects
Move ConstraintSpecification trait out of Widget.scala
Merge pull request #16 from NetLogo/wip-typed-chooser

InputBox and Chooser should restrict type rather than use AnyRef
Merge remote-tracking branch 'origin/master' into wip-mitosis-mark-two