Remove .travis.yml

It is still failing https://travis-ci.org/jibran/dynamic_entity_reference/builds/143022426.

There was 1 failure:
1) Drupal\Tests\dynamic_entity_reference\FunctionalJavascript\DynamicEntityReferenceTest::testFieldSettings
Javascript condition met:
jQuery(".ui-autocomplete.ui-menu li.ui-menu-item:visible").length > 0
Failed asserting that false is true.
/home/travis/build/jibran/drupal/core/tests/Drupal/KernelTests/AssertLegacyTrait.php:31
/home/travis/build/jibran/drupal/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php:102
/home/travis/build/jibran/dynamic_entity_reference/tests/src/FunctionalJavascript/DynamicEntityReferenceTest.php:170

I tried so many fixes can be seen at https://travis-ci.org/jibran/dynamic_entity_reference/builds. @larowlan can you suggest something?

I tired following commits all failed:
https://github.com/jibran/dynamic_entity_reference/commit/cdbd2fee979d0f...
https://github.com/jibran/dynamic_entity_reference/commit/f213cf0ec3f873...
https://github.com/jibran/dynamic_entity_reference/commit/81a73493799679...
https://github.com/jibran/dynamic_entity_reference/commit/20f1c477073845...
https://github.com/jibran/dynamic_entity_reference/commit/b13ced207cb525...
https://github.com/jibran/dynamic_entity_reference/commit/26dde7f02700f3...
https://github.com/jibran/dynamic_entity_reference/commit/26c55ed9587d4c...
https://github.com/jibran/dynamic_entity_reference/commit/c1995a36400c2a...
https://github.com/jibran/dynamic_entity_reference/commit/f58af7744ea7d8...
https://github.com/jibran/dynamic_entity_reference/commit/c205dcc1bf4ee1...
https://github.com/jibran/dynamic_entity_reference/commit/6a09e208f35010...

Created https://github.com/jibran/dynamic_entity_reference/pull/2 for 8.x-1.x and https://github.com/jibran/dynamic_entity_reference/pull/3 for 8.x-2.x

The reason other than stated in #2722749: Add .travis.yml for github repo was to make sure 2.x upgrade path works on every supported PHP and SQL version. DrupalCI only allows limited versions so the idea was to run tests on all the known versions on Travis but now that upgrade path has been there for almost more than a year and I haven't seen any issue popping up in the queue related to the upgrade path I think we can remove it.

#2771389-2: Test DER on all available versions of SQL was the dream. :D

.travis.yml file has been removed.