Merge pull request #398 from hynek/fix-build

Let’s try to fix the build once again, shall we?
diff --git a/tox.ini b/tox.ini
index fcf7095..3970462 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,12 +7,13 @@
 passenv = ARCHFLAGS CFLAGS LC_ALL LDFLAGS PATH LD_LIBRARY_PATH TERM
 deps =
     coverage
-    pytest>=2.8.2
+    pytest>=2.8.2,!=2.8.4
     cryptographyMaster: git+https://github.com/pyca/cryptography.git
 setenv =
     # Do not allow the executing environment to pollute the test environment
     # with extra packages.
     PYTHONPATH=
+    PIP_NO_BINARY=cryptography
 commands =
     openssl version
     python -c "import OpenSSL.SSL; print(OpenSSL.SSL.SSLeay_version(OpenSSL.SSL.SSLEAY_VERSION))"