[ruby/erb] Use libexec same as ruby core repository

https://github.com/ruby/erb/commit/660255cf24

lib/erb.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.files         = Dir.chdir(File.expand_path("..", __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
-  spec.bindir        = "exe"
+  spec.bindir        = "libexec"
   spec.executables   = ["erb"]
   spec.require_paths = ["lib"]
 end