ui: require yarn >= 1.7.0

Builds before the upgrade failed mysteriously.
@benesch (cc) suspects a bug in older versions of yarn.

```
$ make build
Running make with -j8
GOPATH set to /Users/tschottdorf/go
build/node-run.sh -C pkg/ui yarn install --offline
(echo "// Code generated by goyacc. DO NOT EDIT."; \
     echo "// GENERATED FILE DO NOT EDIT"; \
     cat pkg/sql/parser/gen/sql.go.tmp | \
     sed -E 's/^const ([A-Z][_A-Z0-9]*) =.*$/const \1 = lex.\1/g') > pkg/sql/parser/sql.go.tmp || rm pkg/sql/parser/sql.go.tmp
(echo "// Code generated by make. DO NOT EDIT."; \
     echo "// GENERATED FILE DO NOT EDIT"; \
     echo; \
     echo "package lex"; \
     echo; \
     grep '^const [A-Z][_A-Z0-9]* ' pkg/sql/parser/gen/sql.go.tmp) > pkg/sql/lex/tokens.go.tmp || rm pkg/sql/lex/tokens.go.tmp
mv -f pkg/sql/lex/tokens.go.tmp pkg/sql/lex/tokens.go
mv -f pkg/sql/parser/sql.go.tmp pkg/sql/parser/sql.go
yarn install v1.6.0
(node:50064) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
[1/5] Validating package.json...
go install -v docgen
bin/prereqs ./pkg/cmd/docgen > bin/docgen.d.tmp
[2/5] Resolving packages...
mv -f bin/docgen.d.tmp bin/docgen.d
[3/5] Fetching packages...
build/node-run.sh pkg/ui/bin/gen-protobuf-cli-deps.js > pkg/ui/node_modules/protobufjs/cli/package.json
bash: pkg/ui/node_modules/protobufjs/cli/package.json: No such file or directory
make: *** [pkg/ui/yarn.installed] Error 1
make: *** Waiting for unfinished jobs....
```

Release note (general change): the build now requires yarn at version
1.7.0 or above.

CONTRIBUTING.md

@@ -43,7 +43,7 @@ new engineers.
      - GNU Make (3.81+ is known to work)
      - CMake 3.1+
      - Autoconf 2.68+
-     - NodeJS 6.x and Yarn 1.0+
+     - NodeJS 6.x and Yarn 1.7+
 
    Note that at least 4GB of RAM is required to build from source and run tests.
 

pkg/ui/README.md

@@ -16,7 +16,7 @@ you'll be able to access the UI at <http://localhost:8080>.
 Our UI is compiled using a collection of tools that depends on
 [Node.js](https://nodejs.org/) and are managed with
 [Yarn](https://yarnpkg.com), a package manager that offers more deterministic
-package installation than NPM. NodeJS 6.x and Yarn 0.22.0 are known to work.
+package installation than NPM. NodeJS 6.x and Yarn 1.7.0 are known to work.
 [Chrome](https://www.google.com/chrome/), Google's internet browser. Unit tests
 are run using Chrome's "Headless" mode.
 

pkg/ui/package.json

@@ -111,6 +111,6 @@
   },
   "engines": {
     "node": ">=6",
-    "yarn": ">=1.0.0"
+    "yarn": ">=1.7.0"
   }
 }