misc: Fix Travis conditional build

[rhcarvalho]

Not all environment variables that Travis set in build time are
available to conditional build evaluation.

Instead of using environment variables, we can condition on the fork
attribute, which is false for branches in the main repo, and true
for PRs sent from forks.

Reference:
https://docs.travis-ci.com/user/conditions-v1#integration

[rhcarvalho]

with browserstack

This is a build from a branch in the main repo to show the condition being true:

https://github.com/getsentry/sentry-javascript/runs/667219825

without browserstack

This PR is intentionally from a fork to test the condition being false:

https://github.com/getsentry/sentry-javascript/runs/667218127

[rhcarvalho]

Original attempt at fixing the conditional:

Not all environment variables that Travis set in build time are
available to conditional build evalution.

Apart from a list of known attributes, only environment variables from
.travis.yml and repository settings are available and can be matched
using env(FOO).

We then condition the build to the presence of the BROWSERSTACK_USERNAME
variable. Note that 'env(BROWSERSTACK_USERNAME) IS present' is also a
possible solution, though more verbose.

Reference:
https://docs.travis-ci.com/user/conditions-v1#integration

env(BROWSERSTACK_USERNAME) didn't work, it was always true.

Checking the fork attribute works.

[rhcarvalho]

Going to merge this, travis failures are unrelated.