Update vendored dependencies. (dgraph-io#3357)

* Vendor in external dependencies that are externally referenced.

$ govendor list | awk '$1 ~ "e"' | awk '{ printf("govendor update %s/...\n", $2) }' | bash -x
+ govendor update github.com/apache/thrift/lib/go/thrift/...
+ govendor update github.com/dgraph-io/dgo/x/...
+ govendor update github.com/dgrijalva/jwt-go/...
+ govendor update github.com/dustin/go-humanize/english/...
+ govendor update github.com/golang/protobuf/jsonpb/...
+ govendor update github.com/golang/protobuf/ptypes/...
+ govendor update github.com/golang/protobuf/ptypes/any/...
+ govendor update github.com/golang/protobuf/ptypes/duration/...
+ govendor update github.com/golang/protobuf/ptypes/struct/...
+ govendor update github.com/golang/protobuf/ptypes/timestamp/...
+ govendor update github.com/google/uuid/...
+ govendor update go.opencensus.io/plugin/ocgrpc/...
+ govendor update go.opencensus.io/trace/propagation/...
+ govendor update go.opencensus.io/zpages/...
+ govendor update go.opencensus.io/zpages/internal/...
+ govendor update golang.org/x/crypto/ssh/terminal/...
+ govendor update golang.org/x/net/context/...
+ govendor update golang.org/x/net/http2/...
+ govendor update golang.org/x/net/http2/hpack/...
+ govendor update golang.org/x/net/internal/timeseries/...
+ govendor update golang.org/x/net/trace/...
+ govendor update golang.org/x/sync/semaphore/...
+ govendor update golang.org/x/sys/windows/...
+ govendor update google.golang.org/api/support/bundler/...
+ govendor update google.golang.org/genproto/googleapis/rpc/status/...
+ govendor update google.golang.org/grpc/...
+ govendor update google.golang.org/grpc/balancer/...
+ govendor update google.golang.org/grpc/balancer/base/...
+ govendor update google.golang.org/grpc/balancer/roundrobin/...
+ govendor update google.golang.org/grpc/binarylog/grpc_binarylog_v1/...
+ govendor update google.golang.org/grpc/codes/...
+ govendor update google.golang.org/grpc/connectivity/...
+ govendor update google.golang.org/grpc/credentials/...
+ govendor update google.golang.org/grpc/credentials/internal/...
+ govendor update google.golang.org/grpc/encoding/...
+ govendor update google.golang.org/grpc/encoding/gzip/...
+ govendor update google.golang.org/grpc/encoding/proto/...
+ govendor update google.golang.org/grpc/grpclog/...
+ govendor update google.golang.org/grpc/health/...
+ govendor update google.golang.org/grpc/health/grpc_health_v1/...
+ govendor update google.golang.org/grpc/internal/...
+ govendor update google.golang.org/grpc/internal/backoff/...
+ govendor update google.golang.org/grpc/internal/balancerload/...
+ govendor update google.golang.org/grpc/internal/binarylog/...
+ govendor update google.golang.org/grpc/internal/channelz/...
+ govendor update google.golang.org/grpc/internal/envconfig/...
+ govendor update google.golang.org/grpc/internal/grpcrand/...
+ govendor update google.golang.org/grpc/internal/grpcsync/...
+ govendor update google.golang.org/grpc/internal/syscall/...
+ govendor update google.golang.org/grpc/internal/transport/...
+ govendor update google.golang.org/grpc/keepalive/...
+ govendor update google.golang.org/grpc/metadata/...
+ govendor update google.golang.org/grpc/naming/...
+ govendor update google.golang.org/grpc/peer/...
+ govendor update google.golang.org/grpc/resolver/...
+ govendor update google.golang.org/grpc/resolver/dns/...
+ govendor update google.golang.org/grpc/resolver/passthrough/...
+ govendor update google.golang.org/grpc/stats/...
+ govendor update google.golang.org/grpc/status/...
+ govendor update google.golang.org/grpc/tap/...

$ govendor update golang.org/x/sys/unix/...

$ govendor update github.com/pkg/errors/...

* Vendor in grpc v1.20.1.

* Vendor in opencensus-go v0.21.0.