Require rx<3 since new version is not compatible (#235)

Rx 3.0 is not compatible with Python 2.7
and does not have used extension methods.

Author: Cito

setup.py

@@ -21,7 +21,7 @@
 
 sys.path[:] = path_copy
 
-install_requires = ["six>=1.10.0", "promise>=2.1", "rx>=1.6.0"]
+install_requires = ["six>=1.10.0", "promise>=2.1", "rx>=1.6,<3"]
 
 tests_requires = [
     "pytest>=3.3,<4.0",