disable pre-dexing for CI builds

.travis.yml

@@ -27,7 +27,6 @@ jdk:
   - oraclejdk8
 
 env:
-  - TRAVIS_BUILD=true
   global:
     MALLOC_ARENA_MAX=2
 

build.gradle

@@ -53,7 +53,7 @@ subprojects {
                     exclude 'META-INF/ASL2.0'
                 }
 
-                if (System.getenv('TRAVIS_BUILD') == 'true') {
+                if (System.getenv('TRAVIS') == 'true') {
                     System.out.println("Is travis build, disable pre-dexing")
                     dexOptions {
                         preDexLibraries = false;