Update README.md

Author: Gaika

README.md

@@ -12,13 +12,11 @@ Install ArrayFire library: either [download a binary](http://arrayfire.com/downl
 In Julia v0.5 and v0.6:
 ```julia
 Pkg.add("ArrayFire")
-Pkg.checkout("ArrayFire")
 ```
 
 In Julia v0.7:
 ```julia
-Pkg.add("ArrayFire")
-Pkg.checkout("ArrayFire", "julia-v0.7")
+] add ArrayFire
 ```
 
 ## Simple Usage