fix: Resolve 'CryptographyDeprecationWarning' (#341)

The 'paramiko' python module depends on the 'cryptography' module. There
is a known 'CryptographyDeprecationWarning' problem (see
paramiko/paramiko#1369) that can be resolved
by installing 'cryptography==2.4.2'.

requirements.txt

@@ -1,6 +1,7 @@
 ansible==2.7.1
 ansible-vault==1.2.0
 click==7.0
+cryptography==2.4.2
 docker==3.5.1
 filelock==3.0.9
 gitpython==2.1.11