fix: prevent duplicates when cookie cannot be set

When analytics.js is loaded inside an iframe in Safari 7 a cookie
cannot be set, this fix ensures that .anonymousId() returns null
Safari 7 disabled 3rd party cookies by default.
Merge pull request #407 from segmentio/prevent/duplicates

fix: prevent duplicates when cookie cannot be set
Release 2.4.3
Release 2.4.4
upgrade integrations to 2.3
Release 2.4.5
Release 2.4.6
Updating analytics.js integrations to 1.3.2
Release 2.4.7
deps: bump cookie
cookie: add URIError test
Release 2.4.8
Updating analytics.js-integrations
Release 2.4.9
support umd
Merge pull request #411 from segmentio/umd

support umd
Release 2.4.10
update integrations
Release 2.4.11
add cross browser tests