Pin requests.py version in requirements.txt
Pin requests.py version in requirements.txt

Latest (2.5.2 & 2.5.3) requests.py interacts badly with pyinstaller.

See
http://stackoverflow.com/questions/28775276/pyinstaller-error-importerror-no-module-named-requests-packages-chardet-sys

Former-commit-id: 43753546cc8dbcd6f77349246c0c449266a7cdde
TrimBiggs/libnetwork-plugin,caseydavenport/libcalico,djosborne/libcalico,Symmetric/libcalico,TrimBiggs/libnetwork-plugin,insequent/libcalico,L-MA/libcalico,projectcalico/libcalico,tomdee/libcalico,projectcalico/libnetwork-plugin,TrimBiggs/libcalico,plwhite/libcalico,alexhersh/libcalico,robbrockbank/libcalico,tomdee/libnetwork-plugin