Avoid SIP-18 warning under 2.11.0-SNAPSHOT

A new SIP-18 warning has entered the arena of staging.scala.

It arises from the fix for SI-7753 typing this application:

    def mkExpr[T : TypeTag](mirror: Mirror)(tree : Tree) : mirror.universe.Expr[T] =
    val consumeExpr = mkExpr[String](currentMirror)(consumeTree)

Because `currentMirror` isn't a stable value, we now use an
uses an existential to capture the argument type and the stability.

This runs afoul of SIP-18:

    [info] [error] /localhome/jenkinsdbuild/workspace/Community-2.11.x-retronym/dbuild-0.7.1-M1/target-0.7.1-M1/project-builds/shapeless-fbbb9950ee5796e84406e391a287c38710f0fa1d/examples/src/main/scala/shapeless/examples/staging.scala:111: inferred existential type mirror.universe.Expr[String] forSome { val mirror: reflect.runtime.universe.Mirror }, which cannot be expressed by wildcards,  should be enabled
    [info] [error] by making the implicit value scala.language.existentials visible.
    [info] [error] This can be achieved by adding the import clause 'import scala.language.existentials'
    [info] [error] or by setting the compiler option -language:existentials.
    [info] [error] See the Scala docs for value scala.language.existentials for a discussion
    [info] [error] why the feature should be explicitly enabled.
    [info] [error]     val consumeExpr = mkExpr[String](currentMirror)(consumeTree)
    [info] [error]

This commit disables the warning with a feature import.

Review by @milessabin