constrain cowabloga to latest cohttp
restrict facebook-sdk to cohttp<0.12.0
restrict irmin to cohttp<0.12.0
restrict opium to cohttp<0.12.0

see rgrinberg/opium#14
restrict vhd-tool to cohttp<0.12.0
restrict xen-api-client to older cohttp only
remove an excess cohttp constraint
added ppx_overload.1.0.1
lwt.2.4.3 doesnt compile on ocaml>=4.02.0
restrict message-switch to cohttp <0.12.0
restrict websocket to cohttp <0.12.0
+cohttp.0.12.0
+conduit.0.6.0 and +conduit.0.6.1
+cowabloga.0.0.8
+xen-gnt.2.0.0
+xen-api-client.0.9.8
+vhd-tool.0.7.2/0.7.6
+tcpip.2.0.0/2.0.1
+mirage.2.0.0 libraries
+github.0.9.0 and 0.9.1