Update Gutenberg ref

Author: Gerardo

build.gradle

@@ -23,7 +23,7 @@ ext {
     automatticAboutVersion = '1.4.0'
     automatticRestVersion = '1.0.8'
     automatticTracksVersion = '5.0.0'
-    gutenbergMobileVersion = 'v1.119.0-alpha2'
+    gutenbergMobileVersion = '6878-988f4ea8258afce00fb01640d3368bb4117e33f2'
     wordPressAztecVersion = 'v2.1.3'
     wordPressFluxCVersion = '2.79.0'
     wordPressLoginVersion = '1.15.0'