fix >5s quick mode crash, resolve bheisler#658

When using quick mode without "--noplot", any benchmark running over 5 seconds will lead to a crash.
This commit fixes this problem by adding a very small amount (that will not be reported due to rounding) in order to work around the gnuplot crash.
See <bheisler#658>.

src/routine.rs

@@ -103,7 +103,8 @@ pub(crate) trait Routine<M: Measurement, T: ?Sized> {
             // Early exit for extremely long running benchmarks:
             if time_start.elapsed() > maximum_bench_duration {
                 let iters = vec![n as f64, n as f64].into_boxed_slice();
-                let elapsed = vec![t_prev, t_prev].into_boxed_slice();
+                // prevent gnuplot bug when all values are equal
+                let elapsed = vec![t_prev, t_prev + 0.000001].into_boxed_slice();
                 return (ActualSamplingMode::Flat, iters, elapsed);
             }