Remove dependency on yarn

.gitattributes

@@ -5,7 +5,6 @@ app/constants.php export-subst
 .docker export-ignore
 .scrutinizer.yml export-ignore
 .travis.yml export-ignore
-yarn.lock export-ignore
 Dockerfile export-ignore
 docker-compose.yml export-ignore
 Makefile export-ignore

Makefile

@@ -6,7 +6,7 @@ clean:
 	@ rm -rf ./node_modules ./bower_components
 
 static: clean
-	@ yarn install || npm install
+	@ npm install
 	@ ./node_modules/.bin/gulp bower
 	@ ./node_modules/.bin/gulp vendor js css
 	@ ./node_modules/.bin/jshint assets/js/{core,components,polyfills}