bumping requests to 2.0.0+ (elastic#587)

requests < 2.0.0 causes the following attribute error: AttributeError: type object 'Session' has no attribute 'prepare_request'

Author: fxdgear

setup.py

@@ -15,7 +15,7 @@
  'urllib3>=1.8, <2.0',
 ]
 tests_require = [
- 'requests>=1.0.0, <3.0.0',
+ 'requests>=2.0.0, <3.0.0',
  'nose',
  'coverage',
  'mock',