language: python
python:
  - "2.6"
  - "2.7"
  - "3.3"
  - "3.4"
  - "3.5"
  - "3.5dev"
  - "nightly"

install: pip install tox-travis
script: tox

language: python
python:
  - "2.6"
  - "2.7"
  - "3.3"
  - "3.4"
  - "3.5"
  - "3.5-dev"
  - "nightly"

install: pip install tox-travis
script: tox

Use 3.5-dev not 3.5dev in Travis config
Use 3.5-dev not 3.5dev in Travis config

Because 3.5dev isn't a thing.