Use SVG for status badges in README

http://shields.io/ provides many standardized badges for OSS. The PNG versions provided by Shields are similar to the images currently used, but the SVG versions added here are more legible on high pixel density displays.
Merge pull request #517 from seanlinsley/patch-1

Use SVG for status badges in README
Refer ActiveRecord::Base from top level namespace
Merge pull request #522 from yuroyoro/arbase_refer_from_toplevel

Refer ActiveRecord::Base from top level namespace
Use sqlite3-ruby 1.3.8, not 1.3.9

This fixes failing specs for rbx. For more details, see: https://github.com/sparklemotion/sqlite3-ruby/issues/122
platforms :ruby means MRI and Rubinius so we don't need :rbx

For more details on how to specify platform, see: http://bundler.io/v1.3/man/gemfile.5.html
Do not use test/unit for AR 4.1 tests