v0.4.0: (MINOR) Fix for AssertEC2TagValue method (#24)
### SUMMARY
* Fixed bug in `aws.AssertEC2TagValue` method.
* Removed deprecated (and broken) `aws.AssertEC2TagValueE` method.
### REMOVED
* The deprecated AssertEC2TagValueE method has been removed. It was broken and would not have worked.
### FIXES
* The AssertEC2TagValue method now correctly includes the specified Instance ID in the filter list. Previously it did not include this, causing it to assert against all instances in a region.
### MISC
* The tests for the `aws.AssertEC2TagValue` method have been updated to use the `gomock` mocks rather than the hand written ones.
### Contribution Checklist
* [X] All changes have been reflected in comparable unit test changes or additions.
* [ ] (N/A) Any interactions with third party clients are done via interface types rather than direct interactions.
* [ ] (N/A) All new functions follow the required naming standard.
* [ ] (N/A) All new functions follow the required signature standards.