don't add hvac as a dependency for hashicorp vault integration

hvac is just based on requests anyways, and it doesn't support half of
what we need (like the SSH secrets engine API)

requirements/requirements.in

@@ -50,4 +50,3 @@ uWSGI==2.0.17
 uwsgitop==0.10.0
 pip==9.0.1
 setuptools==36.0.1
-hvac==0.7.1

requirements/requirements.txt

@@ -45,7 +45,6 @@ django==1.11.16
 djangorestframework-yaml==1.0.3
 djangorestframework==3.7.7
 future==0.16.0            # via django-radius
-hvac==0.7.1
 hyperlink==18.0.0         # via twisted
 idna==2.8                 # via hyperlink, requests
 incremental==17.5.0       # via twisted