Revert electron ~7.0.0-beta.1 to ^6.0.0
Revert electron ~7.0.0-beta.1 to ^6.0.0

The beta was previously needed for ELECTRON_SKIP_BINARY_DOWNLOAD,
see 975ea2a. Electron folks kindly backported that feature to v6.

Ref Level/leveldown#678
Ref electron/electron#20438

Level/leveldown,Level/leveldown,Level/leveldown