requirements: update 'cryptography'

Changelog:
- 2.7 (2019-05-30)
  https://github.com/pyca/cryptography/blob/2.7/CHANGELOG.rst#27---2019-05-30
- 2.6.1 (2019-02-27)
  https://github.com/pyca/cryptography/blob/2.7/CHANGELOG.rst#261---2019-02-27
- 2.6 (2019-02-27)
  https://github.com/pyca/cryptography/blob/2.7/CHANGELOG.rst#26---2019-02-27

Code diff:
pyca/cryptography@2.5...2.7

Author: glarrain

requirements/base.txt

@@ -2,7 +2,7 @@
 # note: it is mandatory to register all dependencies of the required packages.
 
 # Required packages:
-cryptography==2.5
+cryptography==2.7
 google-api-python-client==1.7.8
 google-auth==1.6.3
 requests==2.22.0

setup.py

@@ -24,7 +24,7 @@ def get_version(*file_paths: Sequence[str]) -> str:
 
 # TODO: add reasonable upper-bound per package.
 requirements = [
-    'cryptography>=2.5',
+    'cryptography>=2.7',
     'google-api-python-client>=1.7.8',
     'google-auth>=1.6.3',
     'requests>=2.22.0',