[geventhttpclient] Remove strong dependency to geventhttpclient>=1.0a

We can remove the strong dependency on 1.0a, Deepinder Setia manage this
fix in https://bugs.launchpad.net/opencontrail/+bug/1306715

Refs: http://lists.opencontrail.org/pipermail/dev_lists.opencontrail.org/2014-April/000930.html

And already merged in Juniper/contrail-third-party#16
And for packaging Juniper/contrail-packages#31