Bump go version used in build

Signed-off-by: Bob Shannon <bshannon@palantir.com>

.travis.yml

@@ -2,9 +2,8 @@ sudo: false
 language: go
 
 go:
- - 1.7.x
- - 1.8.x
- - 1.9.x
+ - 1.10.x
+ - 1.x
 
 script:
  - go test -short ./...

api/prometheus/v1/api.go

@@ -20,7 +20,6 @@ package v1
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -147,7 +146,7 @@ type ActiveTarget struct {
 	DiscoveredLabels model.LabelSet      `json:"discoveredLabels"`
 	Labels           model.LabelSet      `json:"labels"`
 	ScrapeURL        *url.URL            `json:"scrapeUrl"`
-	LastError        error               `json:"lastError"`
+	LastError        string              `json:"lastError"`
 	LastScrape       time.Time           `json:"lastScrape"`
 	Health           scrape.TargetHealth `json:"health"`
 }
@@ -188,11 +187,7 @@ func (a *ActiveTarget) UnmarshalJSON(b []byte) error {
 	a.ScrapeURL = url
 	a.LastScrape = v.LastScrape
 	a.Health = v.Health
-	if v.LastError != "" {
-		a.LastError = errors.New(v.LastError)
-	} else {
-		a.LastError = nil
-	}
+	a.LastError = v.LastError
 
 	return err
 }

api/prometheus/v1/api_test.go

@@ -18,7 +18,6 @@ package v1
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -477,7 +476,7 @@ func TestAPIs(t *testing.T) {
 							"job":      "prometheus",
 						},
 						ScrapeURL:  testURL,
-						LastError:  errors.New("error while scraping target"),
+						LastError:  "error while scraping target",
 						LastScrape: testTime.Round(0),
 						Health:     scrape.HealthGood,
 					},