fix development env  req (#761)
fix development env  req (#761)

when i install for development, get an error:
error: Flask-SQLAlchemy 2.1 is installed but Flask-SQLAlchemy==2.0 is required by set(['flask-appbuilder'])
airbnb/superset,nekia/incubator-superset-dev,dmigo/incubator-superset,georgeke/caravel,BRD-CD/superset,wbsljh/caravel,zhouyao1994/incubator-superset,jeromecn/caravel_viz_full,BRD-CD/superset,nekia/incubator-superset-dev,apache/incubator-superset,wbsljh/caravel,wbsljh/caravel-aidp,airbnb/caravel,asdf2014/superset,wbsljh/caravel-aidp,jeromecn/caravel_viz_full,giaosudau/caravel,massgov/incubator-superset,FrederichCheng/incubator-superset,SingTel-DataCo/incubator-superset,wbsljh/caravel-aidp,giaosudau/caravel,giaosudau/caravel,alanmcruickshank/superset-dev,robert-digit/superset,ArijanaBrlek/superset,mistercrunch/panoramix,ArijanaBrlek/superset,robert-digit/superset,robert-digit/superset,robert-digit/superset,apache/incubator-superset,timifasubaa/incubator-superset,massgov/incubator-superset,zhouyao1994/incubator-superset,alanmcruickshank/superset-dev,asdf2014/superset,timifasubaa/incubator-superset,jeromecn/caravel_viz_full,georgeke/caravel,SingTel-DataCo/incubator-superset,asdf2014/superset,zhouyao1994/incubator-superset,airbnb/superset,giaosudau/caravel,nekia/incubator-superset-dev,dmigo/incubator-superset,massgov/incubator-superset,airbnb/caravel,airbnb/caravel,apache/incubator-superset,timifasubaa/incubator-superset,airbnb/superset,SingTel-DataCo/incubator-superset,dmigo/incubator-superset,jeromecn/caravel_viz_full,francisliyy/caravel-aidp,ArijanaBrlek/superset,francisliyy/caravel-aidp,dmigo/incubator-superset,BRD-CD/superset,alanmcruickshank/superset-dev,alanmcruickshank/superset-dev,timifasubaa/incubator-superset,apache/incubator-superset,airbnb/caravel,asdf2014/superset,apache/incubator-superset,wbsljh/caravel,wbsljh/caravel-aidp,mistercrunch/panoramix,francisliyy/caravel-aidp,FrederichCheng/incubator-superset,massgov/incubator-superset,airbnb/superset,francisliyy/caravel-aidp,ArijanaBrlek/superset,georgeke/caravel,zhouyao1994/incubator-superset,nekia/incubator-superset-dev,SingTel-DataCo/incubator-superset,FrederichCheng/incubator-superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,BRD-CD/superset,georgeke/caravel,FrederichCheng/incubator-superset,mistercrunch/panoramix,wbsljh/caravel