Replace gometalinter with GolangCI-Lint

gometalinter is broken @ HEAD, and I looked into why that was. During
that process, I remembered that we took the linting scripts from
skaffold, and found that in skaffold gometalinter was replaced with
GolangCI-Lint:

https://github.com/GoogleContainerTools/skaffold/pull/619

The change made linting in skaffold faster, so I figured instead of
fixing gometalinter it made more sense to remove it and replace it with
GolangCI-Lint for kaniko as well.

pkg/buildcontext/buildcontext.go

@@ -18,8 +18,9 @@ package buildcontext
 
 import (
 	"errors"
-	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 	"strings"
+
+	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 )
 
 // BuildContext unifies calls to download and unpack the build context.

pkg/buildcontext/s3.go

@@ -17,14 +17,15 @@ limitations under the License.
 package buildcontext
 
 import (
+	"os"
+	"path/filepath"
+
 	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 	"github.com/GoogleContainerTools/kaniko/pkg/util"
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/aws/aws-sdk-go/service/s3/s3manager"
-	"os"
-	"path/filepath"
 )
 
 // S3 unifies calls to download and unpack the build context.