Use ANDROID_NDK instead of ANDROID_NDK_HOME (that's what the build script uses)

.travis.yml

@@ -14,4 +14,4 @@ before_install:
   # newest Android NDK
   - wget http://dl.google.com/android/ndk/android-ndk-r10c-linux-x86_64.bin -O ndk.bin
   - 7z x ndk.bin -y > /dev/null
-  - export ANDROID_NDK_HOME=`pwd`/android-ndk-r10c
+  - export ANDROID_NDK=`pwd`/android-ndk-r10c