Update yarn to version 1.15.2

The yarn version we use in CI is quite outdated. This brings up the
problem that it doesn't support semver for git repositories. In order to
fix that problem updating yarn seems to be the right thing to do.

This patch should fix the CI problem caused by the semver git URL.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

.travis.yml

@@ -4,7 +4,7 @@ cache: yarn
 env:
   global:
     - CXX=g++-4.8
-    - YARN_VERSION=1.3.2
+    - YARN_VERSION=1.15.2
 
 jobs:
   include: