Support ruby-jwt 2.0

This version of ruby-jwt requires specification of the algorithm (see
jwt/ruby-jwt#184) for more information.

I've created a PR on signet to. That has to be merged before ruby-jwt
2.0 can be really used (see https://github.com/google/signet/pull/93).

Tested locally against ruby-jwt 2.0 and 1.5.6.

googleauth.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'faraday', '~> 0.12'
   s.add_dependency 'logging', '~> 2.0'
-  s.add_dependency 'jwt', '~> 1.4'
+  s.add_dependency 'jwt', '>= 1.4', '< 3.0'
   s.add_dependency 'memoist', '~> 0.12'
   s.add_dependency 'multi_json', '~> 1.11'
   s.add_dependency 'os', '~> 0.9'

spec/googleauth/service_account_spec.rb

@@ -54,7 +54,7 @@ shared_examples 'jwt header auth' do
       expect(hdr).to_not be_nil
       expect(hdr.start_with?(auth_prefix)).to be true
       authorization = hdr[auth_prefix.length..-1]
-      payload, = JWT.decode(authorization, @key.public_key)
+      payload, = JWT.decode(authorization, @key.public_key, true, algorithm: 'RS256')
       expect(payload['aud']).to eq(test_uri)
       expect(payload['iss']).to eq(client_email)
     end
@@ -135,7 +135,8 @@ describe Google::Auth::ServiceAccountCredentials do
     blk = proc do |request|
       params = Addressable::URI.form_unencode(request.body)
       _claim, _header = JWT.decode(params.assoc('assertion').last,
-                                   @key.public_key)
+                                   @key.public_key, true,
+                                   algorithm: 'RS256')
     end
     stub_request(:post, 'https://www.googleapis.com/oauth2/v4/token')
       .with(body: hash_including(

spec/googleauth/signet_spec.rb

@@ -57,7 +57,8 @@ describe Signet::OAuth2::Client do
     blk = proc do |request|
       params = Addressable::URI.form_unencode(request.body)
       _claim, _header = JWT.decode(params.assoc('assertion').last,
-                                   @key.public_key)
+                                   @key.public_key, true,
+                                   algorithm: 'RS256')
     end
     stub_request(:post, 'https://accounts.google.com/o/oauth2/token')
       .with(body: hash_including(