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/expression.js

@@ -1759,7 +1759,7 @@ export default class ExpressionParser extends LValParser {
   parseArrowExpression(
     node: N.ArrowFunctionExpression,
     params?: ?(N.Expression[]),
-    isAsync?: boolean,
+    isAsync?: boolean = false,
   ): N.ArrowFunctionExpression {
     // if we got there, it's no more "yield in possible arrow parameters";
     // it's just "yield in arrow parameters"