Fix vendoring of go-gelf to point to specific commit ID

Signed-off-by: Ghislain Bourgeois <ghislain.bourgeois@gmail.com>

vendor.conf

@@ -77,7 +77,7 @@ github.com/syndtr/gocapability 2c00daeb6c3b45114c80ac44119e7b8801fdd852
 github.com/golang/protobuf 7a211bcf3bce0e3f1d74f9894916e6f116ae83b4
 
 # gelf logging driver deps
-github.com/Graylog2/go-gelf v2
+github.com/Graylog2/go-gelf ba920adcee0319adcc15f52851f1458f56547975
 
 github.com/fluent/fluent-logger-golang v1.3.0
 # fluent-logger-golang deps