Enable goimports check, and fixes several imports format (#1525)

This fix enables goimports check and fixes several imports format
so that the import sections are prettier, e.g.:
```
 import (
-       "github.com/miekg/dns"
        "regexp"
        "strconv"
        "strings"
+
+       "github.com/miekg/dns"
 )
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 8765bb37c..b57bdffa0 100644
--- a/Makefile
+++ b/Makefile
@@ -78,7 +78,7 @@ gen:
 linter:
 	go get -u github.com/alecthomas/gometalinter
 	gometalinter --install golint
-	gometalinter --deadline=1m --disable-all --enable=gofmt --enable=golint --enable=vet --exclude=^vendor/ --exclude=^pb/ ./...
+	gometalinter --deadline=2m --disable-all --enable=gofmt --enable=golint --enable=vet --enable=goimports --exclude=^vendor/ --exclude=^pb/ ./...
 
 .PHONY: clean
 clean: