Backport v3's readme update on npm install to 2.3
Merge pull request #6967 from twitter/readme_npm_fix

Update readme npm instructions
Fix: close modal when 'backdrop:false'.

    * Added phantomjs to devDependencies
    * Test port now configurable.
Fixes #7034: Error on modal close when 'backdrop:false'.

Adding 'phantomjs' to devDependencies.
Makefile: Test server port configurable (TEST_PORT).