Update installation instructions in README
Update installation instructions in README

* Note that the CLI requires Ruby 1.9.3 or higher
* Suggest running `rbenv rehash` after installing the bitters gem

innovand/bitters,armandocanals/bitters,bodefuwa/bitters,maxx1128/Maxwell-bitters,oponder/bitters,bodefuwa/bitters,greyhwndz/bitters,oponder/bitters,innovand/bitters,craftsmen/bitters,smithdamen/bitters,oponder/bitters,armandocanals/bitters,craftsmen/bitters,dYale/bitters,thoughtbot/bitters,smithdamen/bitters,cllns/bitters,maxx1128/Maxwell-bitters,smithdamen/bitters,cllns/bitters,dYale/bitters,craftsmen/bitters,thoughtbot/bitters,greyhwndz/bitters,bodefuwa/bitters,greyhwndz/bitters,innovand/bitters,armandocanals/bitters,maxx1128/Maxwell-bitters,dYale/bitters,cllns/bitters