Relax dependencies for Rails master compatibility

Rails 6.0 was recently released, and the version on the master branch is
now 6.1.0.alpha. The upper bound on these dependencies currently
prevents applications using this gem from running against Rails master.

db-query-matchers.gemspec

@@ -15,10 +15,10 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['lib/**/*.rb']
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'activesupport', '>= 4.0', "<= 6.0"
+  spec.add_runtime_dependency 'activesupport', '>= 4.0', "< 6.1"
   spec.add_runtime_dependency 'rspec', '~> 3.0'
 
-  spec.add_development_dependency 'activerecord',  '>= 4.0', "<= 6.0"
+  spec.add_development_dependency 'activerecord',  '>= 4.0', "< 6.1"
   spec.add_development_dependency 'sqlite3'
   spec.add_development_dependency "appraisal", "~> 2.0"