Add test for editvars with required strength
Partial fix for adding "required" edit variables
This change was worked out by studying the differences between the java and js  implementations of cassowary

When the tests are run, an assertion occurs in removeConstraintInternal (line 455 - "eVars != null").
If the assertion is commented out, the library appears to work although it may be leaking
Merge branch 'required_edit_fix'
Temporarily remove assertion (may cause leak or other side effects)