doc: update minimum g++ version to 4.9.4

doc: update minimum g++ version to 4.9.4 · nodejs/node@2abaa86

Original file line number	Diff line number	Diff line change
`@@ -65,7 +65,7 @@ Depending on host platform, the selection of toolchains may vary.`
`65`	`65`
`66`	`66`	`#### Unix`
`67`	`67`
`68`		`-* GCC 4.8.5 or newer`
	`68`	`+* GCC 4.9.4 or newer`
`69`	`69`	`* Clang 3.4.2 or newer`
`70`	`70`
`71`	`71`	`#### Windows`
`@@ -80,8 +80,8 @@ Depending on host platform, the selection of toolchains may vary.`
`80`	`80`
`81`	`81`	`Prerequisites:`
`82`	`82`
`83`		-* `gcc` and `g++` 4.8.5 or newer, or
`84`		-* `clang` and `clang++` 3.4.2 or newer
	`83`	+* `gcc` and `g++` 4.9.4 or newer, or
	`84`	+* `clang` and `clang++` 3.4.2 or newer (macOS: latest Xcode Command Line Tools)
`85`	`85`	`* Python 2.6 or 2.7`
`86`	`86`	`* GNU Make 3.81 or newer`
`87`	`87`