Install watchman before running build on Travis

Summary: After the change D19216026 was landed Travis build is failing, because now Flipper requires watchman to operate. To fix this I added a step to install watchman before running Flipper build there.

Reviewed By: jknoxville

Differential Revision: D19262195

fbshipit-source-id: c68327a177e10e07c97a0a4e383c9cead1c5706a

.travis.yml

@@ -32,6 +32,14 @@ matrix:
       node_js:
         - "11"
 
+      before_install:
+        - cd ..
+        - git clone https://github.com/facebook/watchman.git
+        - cd watchman
+        - git checkout v4.9.0
+        - ./autogen.sh && ./configure && make && sudo make install
+        - cd ../flipper
+
       install:
         - yarn