Minor fixes to arcanist cpplint

Summary:
This fixes the regex in "getLintCode..." so that it accepts lint
codes such as build/c++11 which have become valid lint codes
in later versions of cpplint.

It also corrects the install instructions for the linter (Google's
style guide is no longer available on SVN and has been migrated to
Github).

Test Plan:
For the Regex:

• Create an .arclint in a project such as:

{
  "linters": {
    "cpplint": {
      "type": "cpplint",
      "severity": {
        "build/c++11": "advice"
      }
    }
  }
}

• Run arc lint with the existing linter. This should fail. Patch the linter, and this should now be accepted.

For the Instructions

• Verify the download location wget https://raw.github.com/google/styleguide/gh-pages/cpplint/cpplint.py

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T10118

Differential Revision: https://secure.phabricator.com/D15019