Fix data race in AddTime

There was a data race in `Tachymeter.AddTime`.
`m.Count` was atomically incremented, but accessing `m.Times` slice
wasn't. As such, calling `AddTime()` from multiple goroutines triggered
a data race. The higher the number of goroutines and/or the smaller
the value of `m.Size`, the higher the probability of a data race
occurring.

Fixes jamiealquiza#43

Add -race in travis-ci script

.travis.yml

@@ -5,3 +5,7 @@ go:
   - "1.10"
   - "1.11"
   - tip
+
+script:
+  - go test -v -race -cpu=1,2,4 .
+

tachymeter.go

@@ -104,7 +104,10 @@ func (m *Tachymeter) Reset() {
 
 // AddTime adds a time.Duration to Tachymeter.
 func (m *Tachymeter) AddTime(t time.Duration) {
-	m.Times[(atomic.AddUint64(&m.Count, 1)-1)%m.Size] = t
+	m.Lock()
+	m.Times[m.Count%m.Size] = t
+	m.Count++
+	m.Unlock()
 }
 
 // SetWallTime optionally sets an elapsed wall time duration.

tachymeter_test.go

@@ -1,6 +1,7 @@
 package tachymeter_test
 
 import (
+	"sync"
 	"testing"
 	"time"
 
@@ -54,6 +55,36 @@ func TestAddTime(t *testing.T) {
 	}
 }
 
+func TestAddTimeConcurrent(t *testing.T) {
+	const (
+		ngoroutines = 10
+		ntimes      = 100
+	)
+
+	ta := tachymeter.New(&tachymeter.Config{Size: 3})
+
+	start := time.Now()
+
+	var wg sync.WaitGroup
+	wg.Add(ngoroutines)
+
+	for i := 0; i < ngoroutines; i++ {
+		go func() {
+			for j := 0; j < ntimes; j++ {
+				ta.AddTime(time.Millisecond)
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+
+	ta.SetWallTime(time.Since(start))
+
+	if ta.Times[0] != time.Millisecond {
+		t.Fail()
+	}
+}
+
 func TestSetWallTime(t *testing.T) {
 	ta := tachymeter.New(&tachymeter.Config{Size: 3})