no pre-dexing on travis (performance)

build.gradle

@@ -21,6 +21,11 @@ apply plugin: 'findbugs'
 
 ext {
     supportLibraryVersion = '24.1.1'
+
+    travisBuild = System.getenv("TRAVIS") == "true"
+
+    // allows for -Dpre-dex=false to be set
+    preDexEnabled = "true".equals(System.getProperty("pre-dex", "true"))
 }
 
 repositories {
@@ -127,6 +132,11 @@ android {
         debug.setRoot('build-types/debug')
         release.setRoot('build-types/release')
     }
+
+    dexOptions {
+        // Skip pre-dexing when running on Travis CI or when disabled via -Dpre-dex=false.
+        preDexLibraries = preDexEnabled && !travisBuild
+    }
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_7