remove typo

Author: aleneum

.travis.yml

@@ -9,7 +9,7 @@ python:
 install:
   - pip install python-coveralls 'coverage<5'
   - pip install -r requirements.txt
-  - pip install -r requirements_diagrams.txtless
+  - pip install -r requirements_diagrams.txt
   - pip install -r requirements_test.txt
 script: pytest --doctest-modules --cov transitions/
 after_success: