change some code to let ruby 2.1.2 can use no problem

googleauth.gemspec

@@ -25,14 +25,14 @@ Gem::Specification.new do |gem|
   end
   gem.require_paths = ["lib"]
   gem.platform      = Gem::Platform::RUBY
-  gem.required_ruby_version = ">= 2.4.0"
+  gem.required_ruby_version = ">= 2.0"
 
-  gem.add_dependency "faraday", ">= 0.17.3", "< 2.0"
+  gem.add_dependency "faraday", "~> 0.15.4"
   gem.add_dependency "jwt", ">= 1.4", "< 3.0"
   gem.add_dependency "memoist", "~> 0.16"
   gem.add_dependency "multi_json", "~> 1.11"
   gem.add_dependency "os", ">= 0.9", "< 2.0"
-  gem.add_dependency "signet", "~> 0.14"
+  gem.add_dependency "signet"
 
   gem.add_development_dependency "yard", "~> 0.9"
 end

lib/googleauth/application_default.rb

@@ -34,11 +34,11 @@ module Google
   # Module Auth provides classes that provide Google-specific authorization
   # used to access Google APIs.
   module Auth
-    NOT_FOUND_ERROR = <<~ERROR_MESSAGE.freeze
+    NOT_FOUND_ERROR = """~ERROR_MESSAGE.freeze
       Could not load the default credentials. Browse to
       https://developers.google.com/accounts/docs/application-default-credentials
       for more information
-    ERROR_MESSAGE
+    ERROR_MESSAGE"""
 
     module_function
 

lib/googleauth/compute_engine.rb

@@ -35,16 +35,16 @@ module Google
   # Module Auth provides classes that provide Google-specific authorization
   # used to access Google APIs.
   module Auth
-    NO_METADATA_SERVER_ERROR = <<~ERROR.freeze
+    NO_METADATA_SERVER_ERROR = """
       Error code 404 trying to get security access token
       from Compute Engine metadata for the default service account. This
       may be because the virtual machine instance does not have permission
       scopes specified.
-    ERROR
-    UNEXPECTED_ERROR_SUFFIX = <<~ERROR.freeze
+    """
+    UNEXPECTED_ERROR_SUFFIX = """
       trying to get security access token from Compute Engine metadata for
       the default service account
-    ERROR
+    """
 
     # Extends Signet::OAuth2::Client so that the auth token is obtained from
     # the GCE metadata server.

lib/googleauth/signet.rb

@@ -48,7 +48,7 @@ module Signet
       def apply! a_hash, opts = {}
         # fetch the access token there is currently not one, or if the client
         # has expired
-        token_type = target_audience ? :id_token : :access_token
+        token_type = defined?(target_audience) ? :id_token : :access_token
         fetch_access_token! opts if send(token_type).nil? || expires_within?(60)
         a_hash[AUTH_METADATA_KEY] = "Bearer #{send token_type}"
       end