Travis: Conditional android components (#194)

* Travis: Use environment variable in android components

* Travis: add targetSDKVersion to android components

Author: PLNech

.travis.yml

@@ -27,18 +27,11 @@ android:
     - platform-tools
     - tools # Fix SDK Tools https://github.com/travis-ci/travis-ci/issues/6122
     - build-tools-24.0.3
-    - android-16
-    - android-17
-    - android-19
-    - android-22
-    - android-24
+    - android-24 # target SDK version
+    - $ANDROID_TARGET
     - extra-android-support
     - extra-android-m2repository
-    - sys-img-armeabi-v7a-android-16
-    - sys-img-armeabi-v7a-android-17
-    - sys-img-armeabi-v7a-android-19
-    - sys-img-armeabi-v7a-android-22
-    - sys-img-armeabi-v7a-android-24
+    - sys-img-armeabi-v7a-$ANDROID_TARGET
 cache:
   directories:
     - $HOME/.gradle/caches/