Remove "requirements.txt" mention from README
Remove "requirements.txt" mention from README

I noticed that there isn't the file `requirements.txt` was removed when moving to PyPi. Plus there's no such thing as an open-source project without an "update README" PR. ;)
apache/incubator-superset,wbsljh/caravel-aidp,zhouyao1994/incubator-superset,jeromecn/caravel_viz_full,SingTel-DataCo/incubator-superset,robert-digit/superset,georgeke/caravel,asdf2014/superset,wbsljh/caravel,apache/incubator-superset,timifasubaa/incubator-superset,tienhv/caravel,airbnb/superset,nekia/incubator-superset-dev,zhouyao1994/incubator-superset,BRD-CD/superset,giaosudau/caravel,airbnb/superset,jeromecn/caravel_viz_full,georgeke/caravel,wbsljh/caravel,francisliyy/caravel-aidp,mistercrunch/panoramix,timifasubaa/incubator-superset,robert-digit/superset,tienhv/caravel,dmigo/incubator-superset,tienhv/caravel,FrederichCheng/incubator-superset,francisliyy/caravel-aidp,asdf2014/superset,BRD-CD/superset,FrederichCheng/incubator-superset,georgeke/caravel,BRD-CD/superset,woel0007/caravel,massgov/incubator-superset,SingTel-DataCo/incubator-superset,zhouyao1994/incubator-superset,alanmcruickshank/superset-dev,dmigo/incubator-superset,airbnb/caravel,alanmcruickshank/superset-dev,airbnb/caravel,woel0007/caravel,nekia/incubator-superset-dev,asdf2014/superset,SingTel-DataCo/incubator-superset,wbsljh/caravel-aidp,jeromecn/caravel_viz_full,massgov/incubator-superset,BRD-CD/superset,timifasubaa/incubator-superset,apache/incubator-superset,SingTel-DataCo/incubator-superset,airbnb/superset,massgov/incubator-superset,dmigo/incubator-superset,francisliyy/caravel-aidp,ArijanaBrlek/superset,mistercrunch/panoramix,asdf2014/superset,giaosudau/caravel,woel0007/caravel,ArijanaBrlek/superset,zhouyao1994/incubator-superset,timifasubaa/incubator-superset,francisliyy/caravel-aidp,robert-digit/superset,FrederichCheng/incubator-superset,giaosudau/caravel,massgov/incubator-superset,dmigo/incubator-superset,jeromecn/caravel_viz_full,airbnb/caravel,wbsljh/caravel-aidp,zhouyao1994/incubator-superset,nekia/incubator-superset-dev,FrederichCheng/incubator-superset,wbsljh/caravel,mistercrunch/panoramix,woel0007/caravel,alanmcruickshank/superset-dev,ArijanaBrlek/superset,robert-digit/superset,tienhv/caravel,wbsljh/caravel-aidp,nekia/incubator-superset-dev,wbsljh/caravel,ArijanaBrlek/superset,airbnb/caravel,georgeke/caravel,apache/incubator-superset,giaosudau/caravel,mistercrunch/panoramix,alanmcruickshank/superset-dev,airbnb/superset,apache/incubator-superset