feat(ci): Only run test-xbrowser once (#139)

## Summary

- Only run cross-browser tests once, rather than _n_ times for each Node.js version in the Travis CI matrix

This should reduce contention in both Travis CI and BrowserStack.

## Test plan
CI.

Author: spencerwilson-optimizely

.travis.yml

@@ -15,9 +15,13 @@ env:
   - secure: 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
   # BROWSER_STACK_ACCESS_KEY
   - secure: 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
+matrix:
+  include:
+  - node_js: '10.0'
+    script: npm run test-xbrowser
 before_install: cd packages/optimizely-sdk
 install: npm install
 addons:
   srcclr: true
-script: npm run test-xbrowser
+script: npm test
 after_success: npm run coveralls