Only include specific file extensions in gemspec
Only include specific file extensions in gemspec

The previous gemspec was pretty loose, and I could see unexpected
files slipping in by accident.  Change things so that only
expected file extensions appear in the gem.

nicohvi/sequel,uhoh-itsmaciek/sequel,dennis-d/custom_sequel,yazshel/sequel,nju520/sequel,pfarrell/sequel,knut2/sequel,offshore-safety/sequel,bigfix/sequel,uhoh-itsmaciek/sequel,reubano/sequel,freshpay/sequel,t-morgan/sequel,reubano/sequel,davidbegin/sequel,chef/sequel,gferguson-gd/sequel,davidbegin/sequel,nicohvi/sequel,mpalmer/sequel,knut2/sequel,seeARMS/sequel,t-morgan/sequel,beni55/sequel,offshore-safety/sequel,Marketcircle/sequel,mpalmer/sequel,beni55/sequel,seeARMS/sequel,offshore-safety/sequel,yazshel/sequel,pfarrell/sequel,nju520/sequel,freshpay/sequel,gferguson-gd/sequel,dennis-d/custom_sequel