Fix "undefined method `IOError'" on bad dir

If you tried to run on a directory that wasn't from a twitter archive you would
get "lib/grailbird_updater.rb:23:in `read_required': undefined method `IOError'
for GrailbirdUpdater::JsFile:Class (NoMethodError)". Now you get the expected
  IOError.
Merge pull request #11 from theevocater/minor_bug_fixes

Fix "undefined method `IOError'" on bad dir