Fix closure-linter download instructions in README.md.
Fix closure-linter download instructions in README.md.

Currently, if Python 3 is the default Python version for easy_install,
it will set up closure-linter to run with Python 3, which will break. So
the advice is to explicitly run with Python 2 and then it will install
the correct version.

R=tapted@chromium.org

Review URL: https://codereview.appspot.com/200740043

jackhou-chromium/bitmapper,jackhou-chromium/bitmapper,jackhou-chromium/bitmapper