change req for snowflake to fix crypto install issue (#612)
change req for snowflake to fix crypto install issue (#612)

From cffi callback <function _verify_callback at 0x06BF2978>:
Traceback (most recent call last):
  File "c:\projects\dbt\.tox\pywin\lib\site-packages\OpenSSL\SSL.py", line 313, in wrapper
    _lib.X509_up_ref(x509)
AttributeError: module 'lib' has no attribute 'X509_up_ref'
From cffi callback <function _verify_callback at 0x06B8CF60>:
nave91/dbt,fishtown-analytics/dbt,analyst-collective/dbt,fishtown-analytics/dbt,fishtown-analytics/dbt,nave91/dbt,analyst-collective/dbt