Added missing buildscript snapshot repository required for GDX-Tools resolution

Build script was failing due to not finding '**com.badlogicgames.gdx:gdx-tools:1.0-SNAPSHOT**'. This was because the buildscript-related section of **build.gradle** did not reference the snapshots repository @ https://oss.sonatype.org/content/repositories/snapshots/.

See https://github.com/libgdx/libgdx/issues/1396