Run syntax checker during travis build

In addition to running unit tests, these additional checks ensure that
certain failures will not make it into the code base unintentionally.

.travis.yml

@@ -4,7 +4,12 @@ python:
 # command to install dependencies
 install:
   - pip install -r requirements.txt
+  - pip install pyflakes
   # Deal with issue on Travis builders re: multiprocessing.Queue :(
   - "sudo rm -rf /dev/shm && sudo ln -s /run/shm /dev/shm"
 # command to run tests
-script: make unit functional
+script:
+  - make unit functional
+  # Check that there are no syntax errors
+  - find lettuce -type f -name "*.py" -print | xargs python -m py_compile
+  - find lettuce -type f -name "*.py" -print | xargs pyflakes