Add defcon to test-requirements.txt

I need some font objects to exercise the fonts_to_quadratic function.
Defcon is still not an install requirement though, as cu2qu should
work with any defcon-like objects (e.g. fontParts?, robofab not so much)

test-requirements.txt

@@ -0,0 +1,3 @@
+coverage
+pytest
+defcon==0.3.5

tox.ini

@@ -3,8 +3,7 @@ envlist = py27, py36, htmlcov
 
 [testenv]
 deps =
-    pytest
-    coverage
+    -rtest-requirements.txt
     -rrequirements.txt
 commands =
     coverage run --parallel-mode -m pytest {posargs}