Update the TensorFlow CI info (swiftlang#33920)

Author: shahmishal

README.md

@@ -30,9 +30,9 @@
 
 | **OS** | **Architecture** | **Build** |
 |---|:---:|:---:|
-|**[Ubuntu 16.04](https://github.com/apple/swift-community-hosted-continuous-integration/blob/master/nodes/x86_64_ubuntu_16_04_tensorflow.json)** | x86_64 |[![Build Status](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-16.04-tensorflow/lastCompletedBuild/badge/icon)](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-16.04-tensorflow)|
+|**[Ubuntu 18.04](https://github.com/apple/swift-community-hosted-continuous-integration/blob/master/nodes/x86_64_ubuntu_18_04_tensorflow.json)** | x86_64 |[![Build Status](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-18.04-tensorflow/lastCompletedBuild/badge/icon)](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-18.04-tensorflow)|
 |**[macOS 10.13](https://github.com/apple/swift-community-hosted-continuous-integration/blob/master/nodes/x86_64_macos_high_sierra_tensorflow.json)** | x86_64 |[![Build Status](https://ci-external.swift.org/job/oss-swift-RA-macOS-tensorflow/lastCompletedBuild/badge/icon)](https://ci-external.swift.org/job/oss-swift-RA-macOS-tensorflow)|
-|**[Ubuntu 16.04 (GPU)](https://github.com/apple/swift-community-hosted-continuous-integration/blob/master/nodes/x86_64_ubuntu_16_04_tensorflow_gpu.json)** | x86_64 |[![Build Status](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-16.04-tensorflow-gpu/lastCompletedBuild/badge/icon)](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-16.04-tensorflow-gpu)|
+|**[Ubuntu 18.04 (GPU)](https://github.com/apple/swift-community-hosted-continuous-integration/blob/master/nodes/x86_64_ubuntu_18_04_tensorflow_gpu.json)** | x86_64 |[![Build Status](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-18.04-tensorflow-gpu/lastCompletedBuild/badge/icon)](https://ci-external.swift.org/job/oss-swift-RA-linux-ubuntu-18.04-tensorflow-gpu)|
 
 ## Welcome to Swift