Fix: Verify callee is an Identifier when testing CallExpression name (fixes #25)
Fix: Verify callee is an Identifier when testing CallExpression name (fixes #25)

Both `util.isDefineCall` and `util.isRequireCall` could break in the
event that the CallExpression node's callee was something other than an
Identifier.

* Add additional check for "Identifier" callee type before checking name

shane-tomlinson/eslint-plugin-requirejs,cvisco/eslint-plugin-requirejs