New: Travis yarn cache, yarn version spec, node 10 (#674)

.travis.yml

@@ -1,7 +1,9 @@
 language: csharp
+
 os:
   - linux
   - osx
+
 addons:
   apt:
     packages:
@@ -13,10 +15,20 @@ addons:
       - yarn
       - dos2unix
     update: true
+
 solution: src/Lidarr.sln
+
 before_install:
-  - nvm install 8
-  - nvm use 8
+  - nvm install 10
+  - nvm use 10
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.13.0
+  - export PATH=$HOME/.yarn/bin:$PATH
+
+cache:
+  yarn: true
+  directories:
+    - node_modules
+
 script:
   - ./build.sh
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then ./test.sh Mac Unit; fi