v7.0.0-beta.0

• Reintroduce Variance node (#333) (Daniel Tschinder)
• Rename NumericLiteralTypeAnnotation to NumberLiteralTypeAnnotation (#332) (Charles Pick)
• [7.0] Remove ForAwaitStatement, add await flag to ForOfStatement (#349) (Brandon Dail)
• chore(package): update ava to version 0.18.0 (#345) (greenkeeper[bot])
• chore(package): update babel-plugin-istanbul to version 4.0.0 (#350) (greenkeeper[bot])
• Change location of ObjectTypeIndexer to match flow (#228) (Daniel Tschinder)
• Rename flow AST Type ExistentialTypeParam to ExistsTypeAnnotation (#322) (Toru Kobayashi)
• Revert "Temporary rollback for erroring on trailing comma with spread (#154)" (#290) (Daniel Tschinder)
• Remove classConstructorCall plugin (#291) (Brian Ng)
• Update yarn.lock (Daniel Tschinder)
• Update cross-env to 3.x (Daniel Tschinder)
• [7.0] Remove node 0.10, 0.12 and 5 from Travis (#284) (Sergey Rubanov)
• Remove String.fromCodePoint shim (#279) (Mathias Bynens)