cmd/gomobile: bump default iOS minimum version to 7.0

Recent Xcode versions are unable to build C++ files if the minimum
iOS version is below 7.0. Bump the default version to 7.0; it is
ancient and users can override it with -iosversion.

Fixes golang/go#29969

Change-Id: I4e35e021e0fa10c1681df1073efa73700c3aae69
Reviewed-on: https://go-review.googlesource.com/c/164800
Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/cmd/gomobile/build.go b/cmd/gomobile/build.go
index 6f5c7a4..9c9b28d 100644
--- a/cmd/gomobile/build.go
+++ b/cmd/gomobile/build.go
@@ -48,7 +48,7 @@
 are copied into the output.
 
 Flag -iosversion sets the minimal version of the iOS SDK to compile against.
-The default version is 6.1.
+The default version is 7.0.
 
 The -bundleid flag is required for -target ios and sets the bundle ID to use
 with the app.
@@ -244,7 +244,7 @@
 	cmd.flag.StringVar(&buildLdflags, "ldflags", "", "")
 	cmd.flag.StringVar(&buildTarget, "target", "android", "")
 	cmd.flag.StringVar(&buildBundleID, "bundleid", "", "")
-	cmd.flag.StringVar(&buildIOSVersion, "iosversion", "6.1", "")
+	cmd.flag.StringVar(&buildIOSVersion, "iosversion", "7.0", "")
 
 	cmd.flag.BoolVar(&buildA, "a", false, "")
 	cmd.flag.BoolVar(&buildI, "i", false, "")