Clean up a bit of npm scripts

* Add 'flowtype' checking to 'node_package' folder
* Enable flowtype for Authenticity.js
* Enable flowtype for buildConsoleReplay.js
* Enable flowtype for generatorFunction.js

the README instructs the user to install v5 of the the gem, but v6 is already available. When I followed the readme, I was very surprised when I ran npm upgrade and immediately got a new version of the npm package, which made me wonder if I was missing updates in the ruby gem as well.

Fix AssetsPrecompile#symlink_file logic

When we changed this method in #513 we introduced a regression due
to the difference between calling the shell's `ln -s` command and
using Ruby's `File.symlink` command.

Specifically, the former would not error if the symlink already
existed, while the latter did throw an error. Because it is
sometimes the case that the symlink will already exist, throwing
in this case is not desirable.

This should have not been a problem, however, as this scenario
was supposed to have been properly handled, but that code was not
correct. This commit fixes that code.

* Allow filtering react_on_rails specs with :focus
* Fix possible open resource leak in AssetsPrecompile spec