Fixing Travis build (google#640)

Removing ANDROID_NDK_HOME.
Updating buildToolsVersion 26.0.1 -> 26.0.2
Updating Gradle to 3.5.1 to hopefully get past this problem: https://b.corp.google.com/issues/37127636

.travis.yml

@@ -7,9 +7,9 @@ android:
     components:
         - platform-tools
         - tools
-        - build-tools-25.0.0
+        - build-tools-26.0.1
         - android-22
-        - android-25
+        - android-26
         - sys-img-armeabi-v7a-android-22
         - extra-android-m2repository
         - extra-google-m2repository
@@ -27,5 +27,4 @@ before_script:
   - android-wait-for-emulator
   - adb shell input keyevent 82 &
 script:
-  - export ANDROID_NDK_HOME=/usr/local/android-sdk/ndk-bundle
-  - bash ./gradlew build connectedCheck 
+  - bash ./gradlew build connectedCheck

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Thu Mar 16 09:57:17 PDT 2017
+#Thu Aug 24 09:21:35 PDT 2017
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-3.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-3.5.1-all.zip