Fix for AMD & Symbol polyfill race condition

[ndreckshage]

Issue Summary

There is a race condition when using AMD for older browsers that use a Symbol polyfill. https://github.com/rtfeldman/seamless-immutable/blob/7.1.3/src/seamless-immutable.js#L7 this line detects whether a Symbol polyfill is used. However, immutableInit is called outside of define for AMD (https://github.com/rtfeldman/seamless-immutable/blob/7.1.3/src/seamless-immutable.js#L742).

If a polyfill is in a separate bundle than Seamless, there is a race condition, and if Seamless loads before the polyfill, browsers that use the polyfill crash, because they don't break early on React elements.

Fix

• Calls immutableInit inside define, fixing race condition.
• Based on Fix broken build in travis-ci and fix tests #249 fork, because master spec's don't pass in CI.

[crudh]

@ndreckshage looks reasonable. Can you rebase it on current master, or submit a new PR with only your changes? The CI build has been fixed and additional commits were added to it.

And you can either rebuild the conflicting files or leave them out, I will do a build after merging anyway.