travis.yml: Fix build on nightly python

.travis.yml

@@ -6,9 +6,6 @@ sudo: false
 
 matrix:
     fast_finish: true
-    # Nightly python is failing as of 2018-01-26
-    allow_failures:
-    - python: nightly
 
 language: python
 # For a list of available versions, run
@@ -23,6 +20,9 @@ python:
     - pypy3.5-5.8.0
 
 install:
+    # On nightly, upgrade setuptools first to work around
+    # https://github.com/pypa/setuptools/issues/1257
+    - if [[ $TRAVIS_PYTHON_VERSION == 'nightly' ]]; then travis_retry pip install -U setuptools; fi
     - if [[ $TRAVIS_PYTHON_VERSION == 2* ]]; then travis_retry pip install mock monotonic; fi
     - if [[ $TRAVIS_PYTHON_VERSION == 'pypy' ]]; then travis_retry pip install mock; fi
     # TODO(bdarnell): pycares tests are currently disabled on travis due to ipv6 issues.