Add NDK to build.

.travis.yml

@@ -11,5 +11,8 @@ before_install:
   - wget http://dl.google.com/android/android-sdk_r20.0.3-linux.tgz
   - tar -zxf android-sdk_r20.0.3-linux.tgz
   - export ANDROID_HOME=~/builds/square/retrofit/android-sdk-linux
-  - export PATH=${PATH}:${ANDROID_HOME}/tools:${ANDROID_HOME}/platform-tools
+  - wget http://dl.google.com/android/ndk/android-ndk-r8b-linux-x86.tar.bz2
+  - tar -jxf android-ndk-r8b-linux-x86.tar.bz2
+  - export ANDROID_NDK_HOME=~/builds/square/retrofit/android-ndk-r8b
+  - export PATH=${PATH}:${ANDROID_HOME}/tools:${ANDROID_HOME}/platform-tools:${ANDROID_NDK_HOME}
   - android update sdk --filter 1,3 --no-ui --force