ci: remove xvfb initialization

In 4f1973d, we added bootstrap code
to fix #179 and start xvfb for the benefit of the vg/vgx11 backend.

vg/vgx11 has since been removed from gonum/plot.
So remove the xvfb startup as well.

Author: sbinet

.travis.yml

@@ -16,7 +16,6 @@ matrix:
 
 env:
   global:
-   - DISPLAY: ":99.0"
    - GO111MODULE: "on"
 
 # Get coverage tools, and start the virtual framebuffer.
@@ -30,7 +29,6 @@ before_install:
  # Required for coverage.
  - go get golang.org/x/tools/cmd/cover
  - go get github.com/mattn/goveralls
- - sh -e /etc/init.d/xvfb start
 
 # Get deps, build, test, and ensure the code is gofmt'ed.
 # If we are building as gonum, then we have access to the coveralls api key, so we can run coverage as well.