Tighten gem dependencies to use ~>

This is less prone to cause random breakage when new versions of gems are
released.
Add a .ruby-version as this is tested to run on 1.9.3
Add a json dependency to remove a warning when running tests

    You are using an old or stdlib version of json gem
    Please upgrade to the recent version by adding this to your Gemfile:

      gem 'json', '~> 1.7.7'
Remove custom log formatter, make logging inheritable

By inheritable I mean setting Quickeebooks.logger.level will affect the level of
Quickeebooks::Online::Service::ServiceBase.logger and
Quickeebooks::Windows::Service::ServiceBase.logger (unless those loggers have
been overwritten).
Merge pull request #4 from FundingGates/ew-logging-updates

Logging-related updates