Bump Travis TensorFlow nightly URL: 1.2.0 → 1.2.1 (#182)
Bump Travis TensorFlow nightly URL: 1.2.0 → 1.2.1 (#182)

Summary:
Re-application of #100.

Test Plan:
Consider the following command:
```sh
grep -F NIGHTLY_URL= .travis.yml | \
    cut -d \' -f 2 | \
    xargs curl -sI | \
    grep -F 'HTTP/1.1'
```
Before this commit, it outputs:
```
HTTP/1.1 404 Not Found
HTTP/1.1 404 Not Found
```
After this commit, it outputs:
```
HTTP/1.1 200 OK
HTTP/1.1 200 OK
```

wchargin-branch: bump-tf-nightly-to-1.2.1
tensorflow/tensorboard,agrubb/tensorboard,shakedel/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,tensorflow/tensorboard,tensorflow/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,ioeric/tensorboard,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,ioeric/tensorboard,agrubb/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,shakedel/tensorboard,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,ioeric/tensorboard,shakedel/tensorboard,shakedel/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,francoisluus/tensorboard-supervise,tensorflow/tensorboard,ioeric/tensorboard,ioeric/tensorboard,ioeric/tensorboard,agrubb/tensorboard