Bump dependency on requests from 2.4.3 to 2.6.1

cwltool itself doesn't depend on requests 2.6.1, but CacheControl does,
and having matching dependencies helps Pip find a correct set of
packages to install (see #872).

setup.py

@@ -50,7 +50,7 @@
       include_package_data=True,
       install_requires=[
           'setuptools',
-          'requests >= 2.4.3',
+          'requests >= 2.6.1',
           'ruamel.yaml >= 0.12.4, <= 0.15.51',
           'rdflib >= 4.2.2, < 4.3.0',
           'shellescape >= 3.4.1, < 3.5',