Update base64 requirement from 0.10.1 to 0.11.0 (#275)

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@v0.10.1...v0.11.0)

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

Cargo.toml

@@ -24,7 +24,7 @@ serde = { version = "1.0.90", features = ["derive"] }
 serde_json = "1.0.39"
 tokio-timer = "0.1"
 url = "2.1.0"
-base64 = "0.10.1"
+base64 = "0.11.0"
 derive_more = "0.15.0"
 # Optional deps
 hyper = { version = "0.12.25", optional = true }