Julia 0.5 fixes

[shashi]

• Get Requires.jl to work properly (UndefVarError: ComposeDiff not defined)

[randyzwitch]

This affects Vega also, as I can't precompile (since Escher won't precompile)

[jingpengw]

I just tested with the examples and got this error in Julia 0.5.

WARNING: Method definition require(Symbol) in module Escher at /Users/jpwu/.julia/v0.5/Requires/src/require.jl:12 overwritten in module Main at /Users/jpwu/.julia/v0.5/Requires/src/require.jl:12.
WARNING: using Compose.px in module Main conflicts with an existing identifier.
WARNING: /Users/jpwu/.julia/v0.5/Escher/examples/boids.jl did not return a function
ERROR (unhandled task failure): ArgumentError: stream is closed or unusable
 in check_open(::TCPSocket) at ./stream.jl:258
 in uv_write(::TCPSocket, ::Ptr{UInt8}, ::UInt64) at ./stream.jl:806
 in flush(::TCPSocket) at ./stream.jl:855
 in locked_write(::TCPSocket, ::Bool, ::String, ::UInt8) at /Users/jpwu/.julia/v0.5/WebSockets/src/WebSockets.jl:142
 in handle_control_frame(::WebSockets.WebSocket, ::WebSockets.WebSocketFragment) at /Users/jpwu/.julia/v0.5/WebSockets/src/WebSockets.jl:248
 in read(::WebSockets.WebSocket) at /Users/jpwu/.julia/v0.5/WebSockets/src/WebSockets.jl:315
 in macro expansion at /Users/jpwu/.julia/v0.5/Escher/src/cli/serve.jl:184 [inlined]
 in (::##23#28{Module,Dict{String,Function},WebSockets.WebSocket,Escher.Window})() at ./task.jl:360