Prittier printing of snapshots (#1190)

This uses a custom snapshot serializer to reduce escaping in snapshot files and
make them easier to read.

Snapshot serializers are documented here:
https://facebook.github.io/jest/docs/configuration.html#snapshotserializers-array-string

tests/flow/node_tests/basic_node_modules/__snapshots__/jsfmt.spec.js.snap

@@ -1,10 +1,10 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`foo.js 1`] = `
-"var x = require('bar.js');
+var x = require('bar.js');
 console.log(x);
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-var x = require(\\"bar.js\\");
+var x = require("bar.js");
 console.log(x);
-"
+
 `;