[#198] Test with/without lxml installed on Travis

Author: Bjwebb

.travis.yml

@@ -5,7 +5,13 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+env:
+  - INSTALL_LXML=true
+  - INSTALL_LXML=false
 # Apparently six must be installed first, otherwise setup.py will be unhappy
-install: pip install six && pip install --upgrade -r requirements_dev.txt
+install:
+  - pip install six
+  - pip install --upgrade -r requirements_dev.txt
+  - if [ \"$INSTALL_LXML\" == \"true\" ]; then pip install lxml; fi
 script: py.test --cov .
 after_success: coveralls