cmd/gomobile: update documentation to reflect Go 1.7 requirement
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 <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile