Rails2.7 segfaults (protocolbuffers#7091)

* Add a test suite for ruby 2.7

* Call BigDecimal() instead of BigDecimal.new()

BigDecimal.new was deprecated in ruby 2.6

* Switch FrozenError expectation to a matcher

The error message for FrozenError changed to include more information
about the mutated object. Switch from an exact match to an aproximate
match (equal => match). This does not change the prefix.

* We can safely ignore newest array methods from ruby 2.7

ruby/tests/common_tests.rb

@@ -1739,7 +1739,7 @@ def test_freeze
     m.freeze
 
     frozen_error = assert_raise(FrozenErrorType) { m.optional_int32 = 20 }
-    assert_equal "can't modify frozen #{proto_module}::TestMessage", frozen_error.message
+    assert_match "can't modify frozen #{proto_module}::TestMessage", frozen_error.message
     assert_equal 10, m.optional_int32
     assert_equal true, m.frozen?
 

ruby/tests/repeated_field_test.rb

@@ -20,6 +20,7 @@ def test_acts_like_an_array
       :iter_for_each_with_index, :dimensions, :copy_data, :copy_data_simple,
       :nitems, :iter_for_reverse_each, :indexes, :append, :prepend]
     arr_methods -= [:union, :difference, :filter!]
+    arr_methods -= [:intersection, :deconstruct] # ruby 2.7 methods we can ignore
     arr_methods.each do |method_name|
       assert m.repeated_string.respond_to?(method_name) == true, "does not respond to #{method_name}"
     end

tests.sh

@@ -436,6 +436,10 @@ build_ruby26() {
   internal_build_cpp  # For conformance tests.
   cd ruby && bash travis-test.sh ruby-2.6.0 && cd ..
 }
+build_ruby27() {
+  internal_build_cpp  # For conformance tests.
+  cd ruby && bash travis-test.sh ruby-2.7.0 && cd ..
+}
 
 build_javascript() {
   internal_build_cpp