Update Sinatra to 1.4.5

Also update backports package, its current version is causing issues on
Ruby 2.1.3
Run specs on Travis on Ruby 2.1.3 as well as 1.9.3

Also use bundler to run the specs
Merge pull request #45 from 5apps/update_sinatra_and_backports

Update Sinatra to 1.4.5

Sinatra 1.4.4 and 1.4.5 have memory improvements (fixed leaks and less allocated objects per request)

Also update backports package, its current version is causing issues on
Ruby 2.1.3