Bump cffi from 1.14.6 to 1.15.0

Bumps [cffi](http://cffi.readthedocs.org) from 1.14.6 to 1.15.0.

---
updated-dependencies:
- dependency-name: cffi
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.in

@@ -1,6 +1,6 @@
 asn1==2.2.0 # asn1 2.3.0 introduces enum34 as a dependency, which causes problems on some envs
 cryptography==2.8.0
-cffi==1.14.6
+cffi==1.15.0
 future==0.18.2
 itsdangerous==2.0.1
 pbr==1.10.0

requirements.txt

@@ -8,7 +8,7 @@ asn1==2.2.0
     # via -r requirements.in
 certifi==2018.11.29
     # via requests
-cffi==1.14.6
+cffi==1.15.0
     # via
     #   -r requirements.in
     #   cryptography