Remove Yarn ignore-engines configuration option in TravisCI configuration once TravisCI is using a newer version of ember-try

[notmessenger]

Due to node engine incompatibility between this repo and ember-run-raf, and because ember-cli/ember-try#135 has not yet been released, temporary changes to the .travis.yml file were made in #128

Once ember-cli/ember-try#135 is merged and released we still have to wait for TravisCI to begin using the new released version. Once they are then remove the changes related to the ignore-engines Yarn configuration from the .travis.yml file

Tracking Status

• add --ignore-engines to yarn default options ember-cli/ember-try#135 is merged
• New version of ember-try is released (containing the changes from add --ignore-engines to yarn default options ember-cli/ember-try#135)
• TravisCI uses release version of ember-try that contains add --ignore-engines to yarn default options ember-cli/ember-try#135
• Remove temporary configuration options from .travis.yml file