Update base64 requirement from 0.12.1 to 0.13.0 in /gremlin-client

Updates the requirements on [base64](https://github.com/marshallpierce/rust-base64) to permit the latest version.
- [Release notes](https://github.com/marshallpierce/rust-base64/releases)
- [Changelog](https://github.com/marshallpierce/rust-base64/blob/master/RELEASE-NOTES.md)
- [Commits](marshallpierce/rust-base64@v0.12.1...v0.13.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

gremlin-client/Cargo.toml

@@ -40,7 +40,7 @@ serde_derive="1.0"
 r2d2 = "0.8.3"
 chrono = "0.4"
 lazy_static = "1.3.0"
-base64 = "0.12.1"
+base64 = "0.13.0"
 native-tls = "0.2.3"
 async-tungstenite = { version = "0.4.0", optional = true, default-features=false}
 async-std =  { version = "1.4.0", optional = true, features = ["unstable","attributes"] }