Use sbt-doge to skipping native on 2.10/2.12.

Scala Native currently only support 2.11. 2.12 support is expected in
the next release but not 2.10. sbt-doge is an sbt plugin that adds two
helpful commands to deal with the crossScalaVersions gaps in the native
project:

- `pls $SCALA_VERSION test`, like `++SCALA_VERSION test` except skip
  tests that don't define that SCALA_VERSION in crossScalaVersions.
- `very test`, runs `test` for all projects for all crossScalaVersions
  defined for each individual project. Same works for publishing.

Author: olafurpg

.travis.yml

@@ -1,6 +1,9 @@
 language: scala
 sudo: required
 dist: trusty
+script:
+  # plz is like ++ except it skips projects that are not defined for that scala version.
+  - sbt "plz $TRAVIS_SCALA_VERSION test"
 jdk:
   - oraclejdk8
 scala:

project/build.sbt

@@ -1,3 +1,4 @@
+addSbtPlugin("com.eed3si9n" % "sbt-doge" % "0.1.5")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.18")
 addSbtPlugin("com.typesafe.sbt" % "sbt-osgi" % "0.8.0")
 addSbtPlugin("org.scala-native" % "sbt-crossproject" % "0.2.0")