Update build dependencies, add Travis build config

[rock3r]

This PR includes the following changes:

• Use the latest Gradle (3.1) and Android Gradle plugin (2.2.2)
• Use the latest Android Build-tools version (25.0.0)
• Use the latest Android version (25) as target and compile version
• Use the latest version of the Support Library (25.0.0) and Play Services (9.8.0)
• Improve the .gitignore (see here for details)
• Make *nix gradlew file executable
• Use latest version of Fabric plugin (1.22.0), Twitter SDK (2.1.1), Crashlytics (2.6.5) and Gson (2.8.0)
• Fix build errors after updating all the things
• Adds Travis yml file that can be used to enable Travis CI on the project (still WIP)

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Why this PR?

The Droidcon Italy conference is looking into using Conffa as the base for our conference app this year too (2017, that is). We'll be going through the codebase in the next few months, updating the code, adding features if needed, etc. We hope contributions back to mainline will be appreciated; if they're not, we'll simply keep working on our fork.

[Nataliya-Kulinenko]

@rock3r Thank you for contribution! We will take a look at this PR ASAP.

[Nataliya-Kulinenko]

@rock3r I'm happy to notify you, that our team lead @TarasKunyk has approved this PR. After QA session it will be merged into master.

[rock3r]

That's great @NataliyaOprysko! We've done quite a lot more cleanup/updating on our fork, let me know if you are interested in those changes too. Happy to jump on a VC to discuss what our plans are, if you want, so we can make this a concerted effort.

[Nataliya-Kulinenko]

@rock3r Sounds great! It would be very interesting to hear your plans and thoughts regarding any further development of Connfa! Let's schedule a call for tomorrow or Wednesday this week, what about 3pm EET?

[rock3r]

Sure, can you DM me your email on Twitter (Lemberg is following me) so I can setup a videocall for tomorrow?

[Nataliya-Kulinenko]

Hi Sebastiano, Removed emails, thank you!