Update build.gradle

[Rudloff]

Hello,

This fixes the following error during build:

A problem occurred configuring root project 'vortaro'.
> Could not resolve all files for configuration ':classpath'.
   > Could not find com.android.tools.build:gradle:3.0.1.
     Searched in the following locations:
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.1/gradle-3.0.1.pom
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.1/gradle-3.0.1.jar
     Required by:
         project :

It's also probably what broke the F-Droid build (#6).

[ianmcxa]

Cool, thanks