Remove usage of xray

- xray was useful during dev testing but is neither a dev or runtime dep
- its presence causes issues for libraries that handle signals themselves

Also
- updates grpc.gemspec with requirements
diff --git a/src/ruby/grpc.gemspec b/src/ruby/grpc.gemspec
index 12d4ab1..ae48213 100755
--- a/src/ruby/grpc.gemspec
+++ b/src/ruby/grpc.gemspec
@@ -13,6 +13,9 @@
   s.description   = 'Send RPCs from Ruby using GRPC'
   s.license       = 'BSD-3-Clause'
 
+  s.required_ruby_version = '>= 2.0.0'
+  s.requirements << 'libgrpc ~> 0.6.0 needs to be installed'
+
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- spec/*`.split("\n")
   s.executables   = `git ls-files -- bin/*.rb`.split("\n").map do |f|
@@ -25,13 +28,12 @@
   s.add_dependency 'googleauth', '~> 0.4'  # reqd for interop tests
   s.add_dependency 'logging', '~> 1.8'
   s.add_dependency 'minitest', '~> 5.4'  # reqd for interop tests
-  s.add_dependency 'xray', '~> 1.1'
 
   s.add_development_dependency 'bundler', '~> 1.9'
   s.add_development_dependency 'rake', '~> 10.4'
   s.add_development_dependency 'rake-compiler', '~> 0.9'
-  s.add_development_dependency 'rubocop', '~> 0.30'
   s.add_development_dependency 'rspec', '~> 3.2'
+  s.add_development_dependency 'rubocop', '~> 0.30'
 
   s.extensions = %w(ext/grpc/extconf.rb)
 end