Update apispec package to 0.39.0

Addresses #3356

New `prance` package is used for `validate_spec`

requirements.txt

@@ -1,4 +1,4 @@
-apispec==0.19.0
+apispec==0.39.0
 cfenv==0.5.2
 invoke==0.15.0
 psycopg2==2.7.1
@@ -25,6 +25,7 @@ git+https://github.com/18F/slate.git
 
 # Marshalling
 flask-apispec==0.6.0.post0
+prance==0.13.1
 marshmallow==2.6.0
 marshmallow-sqlalchemy==0.8.0
 git+https://github.com/jmcarp/marshmallow-pagination@master