Fix coverage support check for older Rubies

Author: robinetmiller

test/json/test_helper.rb

@@ -1,10 +1,13 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
 require 'coverage'
+
+branches_supported = Coverage.respond_to?(:supported?) && Coverage.supported?(:branches)
+
 # Coverage module must be started before SimpleCov to work around the cyclic require order.
 # Track both branches and lines, or else SimpleCov misleadingly reports 0/0 = 100% for non-branching files.
 Coverage.start(lines:    true,
-               branches: Coverage.supported?(:branches))
+               branches: branches_supported)
 
 begin
   require 'simplecov'
@@ -14,7 +17,7 @@
   SimpleCov.start do
     # Enabling both coverage types to let SimpleCov know to output them together in reports
     enable_coverage :line
-    enable_coverage :branch if Coverage.supported?(:branches)
+    enable_coverage :branch if branches_supported
 
     # Can't always trust SimpleCov to find files implicitly
     track_files 'lib/**/*.rb'