vagrant: npm is needed to run jstests
README.md: update to tested Vagrant version
vagrant: add post up message
Use --no-bin-links to turn off symlinking for npm install

There are filesystems without symlinks (Vagrant on Windows Host).
See the error at https://github.com/gratipay/gratipay.com/issues/2971
Upgrade legacy way of calling Grunt as attempt to fix Travis jstest

Travis started to fail after disabling symlinking in #2971 to fix
npm failures to install stuff on filesystems that don't support
symlinks.

CLI Grunt interface is now contained in grunt-cli package.

https://stackoverflow.com/questions/14695845/why-does-grunt-fail
Fix #2971 finally!

The accepted answer on
https://stackoverflow.com/questions/14695845/why-does-grunt-fail
meant to install Grunt globally, and it didn't specify that
`npm run-script grunt` is not automagical, but uses `scripts`
section of Gruntfile.js, which is absent in our project.
Merge pull request #2973 from gratipay/theflow

Enabling `make test` in Vagrant