Update Dockerfile

Dockerfile

@@ -12,7 +12,7 @@ RUN apk add --no-cache \
         libxml2-dev \
         libxslt-dev \
         alpine-sdk && \
-    pip install --no-cache-dir cryptography==2.1.4 && \
+    pip install --no-cache-dir cryptography && \
     apk del \
         libressl-dev \
         musl-dev \