ci: downgrade yarn to v1.0.2

With yarn@1.1.0 `aio/` and some guide examples fail to build. The failures seem
to be caused by something (possibly `@ngtools/webpack`, which is a dependency of
`@angular/cli`) which peer-depends on TypeScript (TS@^2.0.2), getting linked to
a higher version (TS@2.4.1) with yarn@1.1.0, which is stricter than the 2.3.2
version we use on `aio/` (and which that something gets linked to with
yarn@1.0.2).

[This change][1] is what is causing this difference in behavior.

Downgrading yarn to v1.0.2 (same we use on master) for now.

[1]: yarnpkg/yarn#4478

aio/yarn.lock

@@ -2002,7 +2002,7 @@ devtools-timeline-model@1.1.6:
     chrome-devtools-frontend "1.0.401423"
     resolve "1.1.7"
 
-dgeni-packages@^0.21.3:
+dgeni-packages@^0.21.4:
   version "0.21.4"
   resolved "https://registry.yarnpkg.com/dgeni-packages/-/dgeni-packages-0.21.4.tgz#c77a0730b52ac5dbab00f10fae47dd148374aeb4"
   dependencies:

scripts/ci/env.sh

@@ -35,7 +35,7 @@ fi
 #######################
 
 setEnvVar NODE_VERSION 6.9.5
-setEnvVar YARN_VERSION 1.1.0
+setEnvVar YARN_VERSION 1.0.2
 setEnvVar SAUCE_CONNECT_VERSION 4.4.9
 setEnvVar PROJECT_ROOT $(cd ${thisDir}/../..; pwd)