pylint version to 1.9.5; fix test issues

Author: dhalbert

.travis.yml

@@ -23,7 +23,8 @@ deploy:
     tags: true
 
 install:
-  - pip install pylint circuitpython-build-tools Sphinx sphinx-rtd-theme pytest
+  - pip install --force-reinstall pylint==1.9.2
+  - pip install circuitpython-build-tools Sphinx sphinx-rtd-theme pytest
 
 script:
   - py.test

tests/test_stepper.py

@@ -14,6 +14,10 @@ class Coil:
     def __init__(self):
         self._duty_cycle = 0
 
+    @property
+    def frequency(self):
+        return 1500
+
     @property
     def duty_cycle(self):
         return self._duty_cycle