tox.ini: Use pytest-httpbin>=0.0.6

This hopefully fixes SSL timeout errors. 

Fixes #308

tox.ini

@@ -12,7 +12,7 @@ envlist = py26, py27, py34, pypy
 [testenv]
 deps =
     pytest
-    pytest-httpbin
+    pytest-httpbin>=0.0.6
 
 commands =
     py.test --verbose --doctest-modules --basetemp={envtmpdir} {posargs:./tests ./httpie}