travis: Add Xvfb server
travis: Add Xvfb server

Signed-off-by: Michael Jeanson <bd309e36450a88bf12e0e03a6bd0dbd5b2cf425c@efficios.com>

lttng/lttng-scope,lttng/lttng-scope,lttng/lttng-scope,alexmonthy/lttng-scope,alexmonthy/lttng-scope,lttng/lttng-scope,alexmonthy/lttng-scope