fix(config): lock down socket.io to 0.9.15 to prevent diconnect in Chrome

Due to disconnects when using iit and ddescribe, as mentioned in #1082, karma
is getting latest 0.9 version of socket.io which does not play nice with
Chrome when isolating tests with iit and ddescribe. Running tests in isolation
with this version of socket.io causes the browser to disconnect, effectively
making iit and ddescribe unusable.