Update gr.jl inline display to match GR.jl

I wanted to switch from using GR directly to using Plots, but the inline plots were not working for me.

The following change is sufficient to generate inline plots in iterm for me
0f167b2be9/src/GR.jl (L3014)

julia> versioninfo()
Julia Version 0.6.0-pre.beta.187
Commit 55c97fb* (2017-04-17 23:06 UTC)
Platform Info:
  OS: Linux (x86_64-redhat-linux)
  CPU: Intel(R) Xeon(R) CPU E5-2670 v3 @ 2.30GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, haswell)
julia> using TerminalExtensions
julia> using Plots
julia> gr(display_type=:inline);
julia> z=[sin(sqrt(x^2+y^2)) for x in linspace(-2π,2π,50), y in linspace(-2π,2π,50)];
julia> contour(z)

src/backends/gr.jl

@@ -1109,7 +1109,7 @@ function _display(plt::Plot{GRBackend})
         ENV["GKS_FILEPATH"] = filepath
         gr_display(plt)
         GR.emergencyclosegks()
-        content = string("\033]1337;File=inline=1;preserveAspectRatio=0:", base64encode(open(readbytes, filepath)), "\a")
+        content = string("\033]1337;File=inline=1;preserveAspectRatio=0:", base64encode(open(read, filepath)), "\a")
         println(content)
         rm(filepath)
     else