Update docs - you need Go 1.12

With Go 1.10 I get the following error:
```
cmd/common.go:54:7: undefined: bytes.ReplaceAll
```

Closes #45
https://github.com/cuelang/cue/pull/45

GitOrigin-RevId: 194ead393daad955e3b089a2e16e7a72684d1538
Change-Id: I89b1bab837eb1a2bbdfc69c885b8eb982819049d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2061
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/doc/install.md b/doc/install.md
index e7f0287..ba6ada4 100644
--- a/doc/install.md
+++ b/doc/install.md
@@ -6,7 +6,7 @@
 
 ### Prerequisites
 
-Go 1.10 or higher (see below)
+Go 1.12 or higher (see below)
 
 ### Installing CUE
 
@@ -35,4 +35,4 @@
 
 Follow the instructions at  https://golang.org/doc/install#install.
 Make sure the go binary is in your path.
-CUE uses Go modules, so there is no need to set up a GOPATH.
\ No newline at end of file
+CUE uses Go modules, so there is no need to set up a GOPATH.