fix: Replace secured tokens with ENV vars (#700)

## Summary
- Replaced security tokens with ENV vars.

## Test plan
https://app.travis-ci.com/github/optimizely/javascript-sdk/builds/236234712

Author: msohailhussain

.travis.yml

@@ -11,10 +11,6 @@ branches:
   - /^\d+\.\d+\.(\d|[x])
 env:
   global:
-    # BROWSER_STACK_USERNAME
-    - secure: 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
-    # BROWSER_STACK_ACCESS_KEY
-    - secure: V8X4AFnJrROZUjLiZir6X6dhWSDLD47tuXZBiKzUE9UXL7t6FRLewR1douokEOg00KeDW1gN59EVjl7ucE12RJZHR7hM0V3vzuSwke7bDhDwJKjXQk+/hVuNsvoSX+g99siEAZMHiLM0xo4mNytzzzT3P8VN6bo1MuUmz0uoi0W00Ka6qOgQwf+L/pmOnb7QKp5jCt93sSFdZJtERhOrGaOIED2IkRm39/M8TrdUIrTW7SA0fEQvte8Vo8YD3IxFwBElGnM8lzRYyLBVuiO9gp7FMUsh2fA2RB8BPUlPrmwY9yAoW0l72Gg1tDgBC0wmMtplVcyit2XNuf9Py49PzCgM9Bqe6TXmJNOWJTLRojVlYe6aeB5QsFccmwgA79qkt5RS6lVVzCZtaxiRJImM0JciYevoiVJFrWsKo06KWtPc0mf3mHMywEaLe+CWqSod0Yf59y0NqbvRv/PIfJsD8BjV8QbKwneqPkq4eq1OKwZEiHR8eXG16o/i0+MnLyzf8Qr5GSH58t7nM1V9aHrz9VZblHDVEsKdJk/T/WxWH6UkNLpDfU8sK4ntl7jL1851/HqBDjp/lqzxEAZDnFE+E2rxlNLD6nGjmBrqIb1xwQ3blGR0YU5uCvp6P5skkyUgTHPMnLPZ3BW1hT4wv7i/3UNDUSNOMMP+AFupEsQwc5A=
     - SDK=javascript
     - SDK_BRANCH=$TRAVIS_PULL_REQUEST_BRANCH
 before_install: cd packages/optimizely-sdk