Updated gemspec and fixed rake install error
Updated gemspec and fixed rake install error

When running rake install I would get this error:

$ rake install
(in .../sinatra)
updated sinatra.gemspec
rake aborted!
Don't know how to build task 'test/render_backtrace_test.rb'

autopp/sinatra,davydovanton/sinatra,ganmacs/sinatra,howtolearntocode/sinatra,kumji/sinatra,twanies/sinatra,d-bot/sinatra,davydovanton/sinatra,jenalgit/sinatra,RosenDimov/sinatra,trexnix/sinatra,nicoayala/sinatra,gferguson-gd/sinatra,QLGu/sinatra,burningTyger/sinatra,emiliocorpus/sinatra,blambeau/sinatra,miyazawatomoka/sinatra,kumji/sinatra,sethuster/sinatra,jenalgit/sinatra,guidohcosta/sinatra,miyazawatomoka/sinatra,ganmacs/sinatra,defsprite/sinatra,DoraALin/sinatra,cyberid41/sinatra,gferguson-gd/sinatra,burningTyger/sinatra,kyontan/sinatra,eileencodes/sinatra,trexnix/sinatra,eileencodes/sinatra,lastcat/sinatra,autopp/sinatra,sethuster/sinatra,nju520/sinatra,jkowens/sinatra,JonMidhir/sinatra,twanies/sinatra,guidohcosta/sinatra,defsprite/sinatra,cyberid41/sinatra,nju520/sinatra,Rayheng/sinatra,porras/sinatra,ABaldwinHunter/sinatra-classic,Callygraphy/morrissey,JonRowe/sinatra,laplaceliu/sinatra,kyontan/sinatra,nottombrown/old-sinatra,midorineko/sinatra,dariubs/sinatra,ryanshaw/sinatra,marocchino/sinatra,wzheng1/sinatra,ktw1222/sinatra,d-bot/sinatra,JonMidhir/sinatra,JonRowe/sinatra,emiliocorpus/sinatra,RosenDimov/sinatra,nicoayala/sinatra,ksw2599/sinatra,siberianbear/sinatra,jkowens/sinatra,ksw2599/sinatra,frechei/sinatra,burningTyger/sinatra,lastcat/sinatra,ryanshaw/sinatra,sinatra/sinatra,frechei/sinatra,ktw1222/sinatra,jkowens/sinatra,mwpastore/sinatra,howtolearntocode/sinatra,DoraALin/sinatra,Wirachmat/sinatra,mwpastore/sinatra,wzheng1/sinatra,dariubs/sinatra,bmizerany/sinatra,midorineko/sinatra,Rayheng/sinatra,siberianbear/sinatra,laplaceliu/sinatra,nottombrown/old-sinatra,porras/sinatra,kgrz/sinatra,sinatra/sinatra,Wirachmat/sinatra,QLGu/sinatra,mwpastore/sinatra,JonMidhir/sinatra,ABaldwinHunter/sinatra-classic