[PE-1374] Update for Sinatra 1.4.3

Add Sinatra 1.4.3 to Gemfile and update Gemfile.lock. Make changes for Sinatra
1.4.3 compatibility: Sinatra 1.4.3 calls URI::Parser any time URI is called.
`encode` is not a method on URI::Parser, so we have to use `escape` instead.
Merge pull request #19 from rlinehan/update_sinatra

[PE-1374] Update for Sinatra 1.4.3