Use init_once around library initialization (#199)

* Raise minimum required version of cffi

Require the fix release of 1.4, which is the first version providing
the init_once() call.

* Wrap library initialization whith cff.init_once()

and use the correct test as specified in libsodium docs. See #187

setup.py

@@ -46,8 +46,8 @@
             "upgrade PyPy to use this library."
         )
 else:
-    requirements.append("cffi>=1.1.0")
-    setup_requirements.append("cffi>=1.1.0")
+    requirements.append("cffi>=1.4.1")
+    setup_requirements.append("cffi>=1.4.1")
 
 
 def here(*paths):

src/nacl/bindings/sodium_core.py

@@ -13,14 +13,18 @@
 # limitations under the License.
 from __future__ import absolute_import, division, print_function
 
-from nacl._sodium import lib
+from nacl._sodium import ffi, lib
 from nacl.exceptions import CryptoError
 
 
+def _sodium_init():
+    if lib.sodium_init() == -1:
+        raise CryptoError("Could not initialize sodium")
+
+
 def sodium_init():
     """
     Initializes sodium, picking the best implementations available for this
     machine.
     """
-    if lib.sodium_init() != 0:
-        raise CryptoError("Could not initialize sodium")
+    ffi.init_once(_sodium_init, "libsodium")