Add scalatra-scalatest dependency to Yeoman-generated apps

The Yeoman generator template didn't include a dependency on scalatra-scalatest. 
It's a `provided` dependency of skinny-test, so it needs to be included explicitly.

````
mkdir skinny-sandbox
cd $_
yo skinny
./skinny test
````

````
[error] /Users/chris/code/skinny-sandbox/src/test/scala/controller/RootControllerSpec.scala:3: object test is not a member of package org.scalatra
[error] import org.scalatra.test.scalatest._
[error]                     ^
[error] /Users/chris/code/skinny-sandbox/src/test/scala/controller/RootControllerSpec.scala:6: not found: type ScalatraFlatSpec
[error] class RootControllerSpec extends ScalatraFlatSpec with SkinnyTestSupport {
[error]                                  ^
[error] /Users/chris/code/skinny-sandbox/src/test/scala/controller/RootControllerSpec.scala:8: not found: value addFilter
[error]   addFilter(Controllers.root, "/*")
[error]   ^
[error] /Users/chris/code/skinny-sandbox/src/test/scala/controller/RootControllerSpec.scala:10: not found: value it
[error]   it should "show top page" in {
[error]   ^
[error] four errors found
[error] (dev/test:compile) Compilation failed
````