deps: update grpc to 1.27.0, google-auth-library to 0.20.0 (#861)

* update core dependencies

google-http-client to 1.34.1
grpc to 1.27.0
google-auth-library to 0.20.0

* skip http-client

dependencies.properties

@@ -25,7 +25,7 @@ version.gax_httpjson=0.70.1-SNAPSHOT
 # with the sources.
 version.com_google_protobuf=3.11.1
 version.google_java_format=1.1
-version.io_grpc=1.26.0
+version.io_grpc=1.27.0
 
 # Maven artifacts.
 # Note, the actual name of each property matters (bazel build scripts depend on it).
@@ -34,8 +34,8 @@ version.io_grpc=1.26.0
 #   2) Replace all characters which are neither alphabetic nor digits with the underscore ('_') character
 maven.com_google_api_grpc_proto_google_common_protos=com.google.api.grpc:proto-google-common-protos:1.17.0
 maven.com_google_api_grpc_grpc_google_common_protos=com.google.api.grpc:grpc-google-common-protos:1.17.0
-maven.com_google_auth_google_auth_library_oauth2_http=com.google.auth:google-auth-library-oauth2-http:0.19.0
-maven.com_google_auth_google_auth_library_credentials=com.google.auth:google-auth-library-credentials:0.19.0
+maven.com_google_auth_google_auth_library_oauth2_http=com.google.auth:google-auth-library-oauth2-http:0.20.0
+maven.com_google_auth_google_auth_library_credentials=com.google.auth:google-auth-library-credentials:0.20.0
 maven.io_opencensus_opencensus_api=io.opencensus:opencensus-api:0.24.0
 maven.io_opencensus_opencensus_contrib_grpc_metrics=io.opencensus:opencensus-contrib-grpc-metrics:0.24.0
 maven.io_opencensus_opencensus_contrib_http_util=io.opencensus:opencensus-contrib-http-util:0.24.0