Enable batchMode on scala.js Optimizer

The `batchMode` should help reduce the memory requirement to compile
scala.js code as per
scala-js/scala-js#1788 (comment)

Implementation details: `botBuild` is a sbt setting that will be turned
on on travisCi so that it will not interfere with local dev experience.

.travis.yml

@@ -21,6 +21,7 @@ env:
   global:
   - secure: Kf44XQFpq2QGe3rn98Dsf5Uz3WXzPDralS54co7sqT5oQGs1mYLYZRYz+I75ZSo5ffZ86H7M+AI9YFofqGwAjBixBbqf1tGkUh3oZp2fN3QfqzazGV3HzC+o41zALG5FL+UBaURev9ChQ5fYeTtFB7YAzejHz4y5E97awk934Rg=
   - secure: QbNAu0jCaKrwjJi7KZtYEBA/pYbTJ91Y1x/eLAJpsamswVOvwnThA/TLYuux+oiZQCiDUpBzP3oxksIrEEUAhl0lMtqRFY3MrcUr+si9NIjX8hmoFwkvZ5o1b7pmLF6Vz3rQeP/EWMLcljLzEwsrRXeK0Ei2E4vFpsg8yz1YXJg=
+  - botBuild=true
 cache:
   directories:
   - $HOME/.sbt/0.13/dependency

build.sbt

@@ -5,6 +5,8 @@ import sbtunidoc.Plugin.UnidocKeys._
 import ReleaseTransformations._
 import ScoverageSbtPlugin._
 
+lazy val botBuild = settingKey[Boolean]("Jenkins/TravisCI build mode")
+
 lazy val scoverageSettings = Seq(
   ScoverageKeys.coverageMinimum := 60,
   ScoverageKeys.coverageFailOnMinimum := false,
@@ -55,7 +57,11 @@ lazy val commonJsSettings = Seq(
     s"-P:scalajs:mapSourceURI:$a->$g/"
   },
   scalaJSStage in Global := FastOptStage,
-  parallelExecution := false
+  parallelExecution := false,
+  // Only use for scala.js for now
+  botBuild := sys.props.getOrElse("botBuild", default="false") == "true",
+  // batch mode decreases the amount of memory needed to compile scala.js code
+  scalaJSOptimizerOptions := scalaJSOptimizerOptions.value.withBatchMode(botBuild.value)
 )
 
 lazy val commonJvmSettings = Seq(