Merge pull request #192 from SunPj/SunPj-Last-Modified-HTTP-header

Fixes "Main page caching doesn't work properly"

build.sbt

@@ -1,11 +1,14 @@
+import sbt.Package.FixedTimestamp
+
 name := """play-scala-react-seed"""
 
 version := "1.0-SNAPSHOT"
 
 lazy val root = (project in file("."))
   .enablePlugins(PlayScala)
   .settings(
-    watchSources ++= (baseDirectory.value / "public/ui" ** "*").get
+    watchSources ++= (baseDirectory.value / "public/ui" ** "*").get,
+    ThisBuild / packageOptions += FixedTimestamp(Package.keepTimestamps)
   )
 
 resolvers += Resolver.sonatypeRepo("snapshots")