Minor spelling tweaks
Fix broken links in Readme
Moved sell_in specs into invariants.
added a Gemfile for dependencies to ease installation for ruby beginners
Merge pull request #3 from kaichanvong/master

Fix link in readme.
Merge pull request #4 from bkleinen/dependencies

added a Gemfile for dependencies to ease installation for ruby beginners
Merge pull request #2 from koriroys/errata

Minor spelling/grammar tweaks