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

@@ -26,6 +26,7 @@ env:
   - secure: Kf44XQFpq2QGe3rn98Dsf5Uz3WXzPDralS54co7sqT5oQGs1mYLYZRYz+I75ZSo5ffZ86H7M+AI9YFofqGwAjBixBbqf1tGkUh3oZp2fN3QfqzazGV3HzC+o41zALG5FL+UBaURev9ChQ5fYeTtFB7YAzejHz4y5E97awk934Rg=
   - secure: QbNAu0jCaKrwjJi7KZtYEBA/pYbTJ91Y1x/eLAJpsamswVOvwnThA/TLYuux+oiZQCiDUpBzP3oxksIrEEUAhl0lMtqRFY3MrcUr+si9NIjX8hmoFwkvZ5o1b7pmLF6Vz3rQeP/EWMLcljLzEwsrRXeK0Ei2E4vFpsg8yz1YXJg=
   - TRAVIS_NODE_VERSION="4"
+  - CATS_BOT_BUILD=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]("Build by TravisCI instead of local dev environment")
+
 lazy val scoverageSettings = Seq(
   ScoverageKeys.coverageMinimum := 60,
   ScoverageKeys.coverageFailOnMinimum := false,
@@ -59,7 +61,11 @@ lazy val commonJsSettings = Seq(
   // Using Rhino as jsEnv to build scala.js code can lead to OOM, switch to PhantomJS by default
   scalaJSUseRhino := false,
   requiresDOM := false,
-  jsEnv := NodeJSEnv().value
+  jsEnv := NodeJSEnv().value,
+  // Only used for scala.js for now
+  botBuild := sys.props.getOrElse("CATS_BOT_BUILD", 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(