Remove Java 8 features from geojson module

[tobrun]

Request to remove java 8 from geojson module and as result remove the static method decleration of toJson on the Geometry interface, see here. Since the method isn't public can we patch this up without breaking semver?

cc @osana @zugaldia

[zugaldia]

@osana could you run with this? Ideally this would be part of the upcoming f Maps SDK release.

[osana]

@zugaldia @tobrun By making GeoJson and abstract class instead of an interface we do not need to remove any methods. Please see the PR if you are Ok with that

[tobrun]

afaik the only thing to address is reverting https://github.com/mapbox/mapbox-java/blob/master/services-geojson/src/main/java/com/mapbox/geojson/Geometry.java#L26 as this is a java 8 specific feature (static methods on interfaces). Next to that we need to set the compile version back to 1.7. I'm not seeing the need for the changes made in #833, those should compile with java 7 without a change?