Update README.md for Ubuntu supported updates (#126)

We now support older Ubuntu releases to accommodate Travis.

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Author: nkreeger

README.md

@@ -11,8 +11,8 @@ actively developing as an open source project.**
 
 TensorFlow.js for Node currently supports the following platforms:
 - Mac OS X CPU (10.12.6 Siera or higher)
-- Linux CPU (Ubuntu 16.04 or higher)
-- Linux GPU (Ubuntu 16.04 or higher and Cuda 9.0 w/ CUDNN v7) ([see installation instructions](https://www.tensorflow.org/install/install_linux))
+- Linux CPU (Ubuntu 14.04 or higher)
+- Linux GPU (Ubuntu 14.04 or higher and Cuda 9.0 w/ CUDNN v7) ([see installation instructions](https://www.tensorflow.org/install/install_linux))
 
 *Other Linux variants might also work but this project matches [core TensorFlow installation requirements](https://www.tensorflow.org/install/install_linux).*