Revert "Revert "Speed up ndk-build from sbt-android""

This reverts commit bcbd860457.

.travis.yml

@@ -50,4 +50,4 @@ install:
       fi
 
 script:
-    - $SBTPATH/sbt native-build android:package-debug
+    - $SBTPATH/sbt -DNPROC=$(nproc) native-build android:package-debug

build.sbt

@@ -8,6 +8,11 @@ name := "shadowsocks"
 version := "3.4.2"
 versionCode := Some(174)
 
+// Speed up ndk-build by reading nproc from environment
+// and default to 8
+val nproc = sys.props.getOrElse("NPROC", default = "8")
+ndkArgs := Seq(s"-j$nproc")
+
 platformTarget := "android-25"
 
 compileOrder := CompileOrder.JavaThenScala