Fix running flow on travis and update flow (#9128)

* Fix running flow on travis and update flow

- ensure bootstrap is run before running flow as we need some generated files for correctly doing typechecks
- ensure that we only ignore the build directory inside the babel folder as currently we ignore everything because travis checks out into ‘/home/travis/build/’

* Fix all flow errors

packages/babel-parser/src/parser/statement.js

@@ -896,9 +896,9 @@ export default class StatementParser extends ExpressionParser {
   parseFunction<T: N.NormalFunction>(
     node: T,
     isStatement: boolean,
-    allowExpressionBody?: boolean,
-    isAsync?: boolean,
-    optionalId?: boolean,
+    allowExpressionBody?: boolean = false,
+    isAsync?: boolean = false,
+    optionalId?: boolean = false,
   ): T {
     const oldInFunc = this.state.inFunction;
     const oldInMethod = this.state.inMethod;