Update README.md badge: Drop prebuilt
Update README.md badge: Drop prebuilt

As documented later in the README.md already, plus the download numbers look bigger too.
the-ress/electron,bpasero/electron,seanchas116/electron,gerhardberger/electron,seanchas116/electron,the-ress/electron,shiftkey/electron,bpasero/electron,thomsonreuters/electron,gerhardberger/electron,electron/electron,bpasero/electron,seanchas116/electron,thomsonreuters/electron,shiftkey/electron,shiftkey/electron,seanchas116/electron,seanchas116/electron,Floato/electron,Floato/electron,thomsonreuters/electron,electron/electron,electron/electron,Floato/electron,shiftkey/electron,shiftkey/electron,the-ress/electron,thomsonreuters/electron,the-ress/electron,Floato/electron,Floato/electron,thomsonreuters/electron,bpasero/electron,gerhardberger/electron,gerhardberger/electron,electron/electron,Floato/electron,the-ress/electron,bpasero/electron,gerhardberger/electron,electron/electron,the-ress/electron,electron/electron,thomsonreuters/electron,gerhardberger/electron,electron/electron,bpasero/electron,shiftkey/electron,the-ress/electron,bpasero/electron,gerhardberger/electron,seanchas116/electron