Reorder link and install steps

Take advantage of Yarn's preservation of links while installing and set up links
first and install second. This should result in a small speedup since we no
longer install the published SDKs just to replace them on the next line.

scripts/travis/build.sh

@@ -9,19 +9,17 @@ set -ev
 RIOT_WEB_DIR=riot-web
 REACT_SDK_DIR=`pwd`
 
+yarn link
+
 scripts/fetchdep.sh vector-im riot-web
+
 pushd "$RIOT_WEB_DIR"
 
-mkdir node_modules
+yarn link matrix-js-sdk
+yarn link matrix-react-sdk
+
 yarn install
 
-# use the version of js-sdk we just used in the react-sdk tests
-rm -r node_modules/matrix-js-sdk
-ln -s "$REACT_SDK_DIR/node_modules/matrix-js-sdk" node_modules/matrix-js-sdk
-
-# ... and, of course, the version of react-sdk we just built
-rm -r node_modules/matrix-react-sdk
-ln -s "$REACT_SDK_DIR" node_modules/matrix-react-sdk
-
 yarn build
+
 popd

scripts/travis/install-deps.sh

@@ -1,11 +1,13 @@
 #!/bin/sh
 
 set -ex
-yarn install
-scripts/fetchdep.sh matrix-org matrix-js-sdk
-rm -r node_modules/matrix-js-sdk || true
-ln -s ../matrix-js-sdk node_modules/matrix-js-sdk
 
-cd matrix-js-sdk
+scripts/fetchdep.sh matrix-org matrix-js-sdk
+
+pushd matrix-js-sdk
+yarn link
+yarn install
+popd
+
+yarn link matrix-js-sdk
 yarn install
-cd ..