Switch from x/net/context to context

Since go 1.7, "context" is a standard package. Since go 1.9,
x/net/context merely provides some types aliased to those in
the standard context package.

The changes were performed by the following script:

for f in $(git ls-files \*.go | grep -v ^vendor/); do
	sed -i 's|golang.org/x/net/context|context|' $f
	goimports -w $f
	for i in 1 2; do
		awk '/^$/ {e=1; next;}
			/\t"context"$/ {e=0;}
			{if (e) {print ""; e=0}; print;}' < $f > $f.new && \
				mv $f.new $f
		goimports -w $f
	done
done

[v2: do awk/goimports fixup twice]
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

cli/command/cli_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"context"
 	"crypto/x509"
 	"os"
 	"runtime"
@@ -17,7 +18,6 @@ import (
 	"github.com/gotestyourself/gotestyourself/env"
 	"github.com/gotestyourself/gotestyourself/fs"
 	"github.com/pkg/errors"
-	"golang.org/x/net/context"
 )
 
 func TestNewAPIClientFromFlags(t *testing.T) {