.travis.yml: remove workaround to install Twisted 13.0 which should n…

…o longer be needed.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

.travis.yml

@@ -8,7 +8,6 @@ before_install:
   - python misc/build_helpers/show-tool-versions.py
   - python -c "import os; print os.statvfs('.')"
 install:
-  - pip install 'twisted>=0.13.0'
   - pip install 'coveralls==0.4.2'
 before_script: python setup.py build
 script: bin/tahoe @coverage run --branch --source=src/allmydata @tahoe --version-and-path debug trial --rterrors --reporter=timing