Update the Android plugin for Gradle

[abdulwd]

Fixes #518

Update the Android plugin for Gradle and add Android SDK components in .travis.yml.

[brijeshshah13]

@mhutti1 @abdulwd Does the project build successfully for you after updating the Android Plugin and Gradle version? I seem to have stuck at Refreshing 'kiwix-android' Gradle project while building after fetching the latest branch.

[harshvardhan-takawale]

@brijeshshah13 I tried building it. It works for me.

[brijeshshah13]

@harshvardhan-takawale Thanks for verifying.

[mhutti1]

Sometimes you need to rebuild the project.

[brijeshshah13]

@mhutti1 I rebuilt nearly 25 times, tried deleting .gradle folder and fresh installing gradle 4-5 times, uninstalled and installed Android Studio around 8 times, still no luck :( But it's fine, I work with Gradle 4.5. I even sent a PR for the same.