[travis] Only run tests on master; ./build.sh.

Don't consider the build broken if a feature branch is broken.

Move the actual build command to a separate script -- easier to read,
easier to maintain, and easier to run by hand.

.travis.yml

@@ -7,4 +7,9 @@ before_install:
 env:
   - TEST_OS=7.1
   - TEST_OS=8.1
-script: xctool -workspace AsyncDisplayKit.xcworkspace -scheme AsyncDisplayKit -sdk iphonesimulator8.1 -destination "platform=iOS Simulator,OS=${TEST_OS},name=iPhone 5" build test
+script: ./build.sh $TEST_OS
+
+# whitelist
+branches:
+  only:
+    - master

build.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+xctool \
+    -workspace AsyncDisplayKit.xcworkspace \
+    -scheme AsyncDisplayKit \
+    -sdk iphonesimulator8.1 \
+    -destination "platform=iOS Simulator,OS=${1},name=iPhone 5" \
+    build test