gemspec: do not depend on bundler gem version

We don't use it directly so we don't need to depend on a specific version.

Author: carlosmn

docurium.gemspec

@@ -20,7 +20,6 @@ Gem::Specification.new do |s|
   s.add_dependency "rugged", "~>0.21"
   s.add_dependency "redcarpet", "~>3.0"
   s.add_dependency "ffi-clang", "~> 0.5"
-  s.add_development_dependency "bundler",   "~>1.0"
   s.add_development_dependency "rake", "~> 12"
   s.add_development_dependency "minitest", "~> 5.11"