cmd/gomobile: update documentation to reflect Go 1.7 requirement

CL 35953 bumped the minimum required version to Go 1.7. Update the
gomobile documentation to reflect that.

Change-Id: I7cd7937471eb16456a51e8f41bae331344860888
Reviewed-on: https://go-review.googlesource.com/40919
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/main.go

@@ -191,7 +191,7 @@ To install:
 	$ go get golang.org/x/mobile/cmd/gomobile
 	$ gomobile init
 
-At least Go 1.5 is required.
+At least Go 1.7 is required.
 For detailed instructions, see https://golang.org/wiki/Mobile.
 
 Usage: