Deploy a signed and aligned apk

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In tag v0.7.0 the deployed apk is unsigned and unaligned, failing to install on devices.

Resources:

• https://android.jlelse.eu/using-travisci-to-securely-build-and-deploy-a-signed-version-of-your-android-app-94afdf5cf5b4
• http://www.androiddocs.com/tools/publishing/app-signing.html
• https://github.com/tdillon/android

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Build errors. This is the issue: https://stackoverflow.com/questions/53996492/an-error-for-signingconfig-container-type-org-gradle-api-internal-factorynameddo/53996760

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Only add release signing config on travis builds: https://stackoverflow.com/questions/29919066/what-is-the-best-practice-to-use-keystores-to-sign-release-version-of-an-android

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./gradlew assembleRelease already produces the signed and aligned apk, if configured properly!

Resource: https://code-examples.net/en/q/13f3c70

Remove unneeded Travis commands!